feat(views): Phase 5i slice E — default view-per-page + opt-out banner

Closes the Phase 5i implementation chain. When `views.is_default_for=<page>`
is set, opening that page with a "clean" URL (no chip params, no
?view=) auto-applies the saved filter + view_type. A "Showing default
view: <name> · clear" banner makes the swap visible and gives the user
a one-click out. Adding any chip param to the URL bypasses the default;
?nodefault=1 is the explicit opt-out for "I want the bare default tree".

New web/views.go: applyDefaultView gates on the param-cleanness check
+ Store.DefaultViewFor lookup. Resolution + view_type revalidation
mirror the slice D ?view=<uuid> path so a kanban-default opened on a
route that doesn't allow kanban falls back cleanly.

handleTree wires it into the existing slice D else-branch (no default
when ?view= is set). DefaultBanner field passes the applied view to
the template for the banner.

Test:
- TestDefaultViewAppliedOnCleanURL — seeds a tree default with
  filter_json={tags:[work]} + view_type=card, then asserts: clean GET /
  applies (card grid + banner with the view's name); ?tag=dev bypasses
  (forest, no banner); ?nodefault=1 opt-out (forest, no banner).

web/server.go

@@ -450,6 +450,7 @@ func (s *Server) handleTree(w http.ResponseWriter, r *http.Request) {
 	filter := ParseTreeFilter(r.URL.Query())
 	viewSet := PageViewTypes("/")
 	view := ParseViewType(r.URL.Query(), viewSet)
+	var defaultBanner *store.View
 	// Phase 5i Slice D: ?view=<uuid> resolves a saved view's filter +
 	// view_type into the current request, overriding URL-only chip state.
 	// Resolution failure (deleted view, malformed payload) is logged and
@@ -462,6 +463,16 @@ func (s *Server) handleTree(w http.ResponseWriter, r *http.Request) {
 		view = viewSet.Resolve(view)
 	} else if err != nil {
 		s.Logger.Warn("applySavedView", "id", r.URL.Query().Get("view"), "err", err)
+	} else {
+		// Phase 5i Slice E: no explicit ?view= → check for a page default.
+		// applyDefaultView returns nil unless the URL is "clean" (no chip
+		// state) AND a default exists for this page.
+		if def, err := s.applyDefaultView(r, "tree", &filter, &view); err == nil && def != nil {
+			view = viewSet.Resolve(view)
+			defaultBanner = def
+		} else if err != nil {
+			s.Logger.Warn("applyDefaultView", "page", "tree", "err", err)
+		}
 	}
 	roots, orphans, total, orphanN, matched := applyTreeFilter(items, filter, linkKinds)
 	counts := computeChipCounts(items, filter, linkKinds, tags)
@@ -492,6 +503,7 @@ func (s *Server) handleTree(w http.ResponseWriter, r *http.Request) {
 		"Kanban":            kanban,
 		"GroupBy":           groupBy,
 		"GroupByChips":      groupByChips,
+		"DefaultBanner":     defaultBanner,
 		// ActiveTags kept for backwards-compat with the old template path; removed
 		// after the template migrates fully.
 		"ActiveTags": filter.Tags,

web/static/style.css

@@ -259,6 +259,15 @@ table.classify input, table.classify select { width: 100%; }
 .kanban-card-title:hover { color: var(--accent); }
 .kanban-card-meta { display: flex; flex-wrap: wrap; gap: 4px; margin: 4px 0 0; font-size: 0.78em; }
 .kanban-empty { padding: 24px; }
+/* Phase 5i Slice E — default-view banner. Sits above the counts line on
+   any Views-supporting page when a default view is auto-applied. */
+.default-banner {
+  background: var(--surface); border: 1px solid var(--border);
+  border-left: 3px solid var(--accent);
+  padding: 6px 10px; border-radius: 4px;
+  font-size: 0.85em; margin: 4px 0 8px;
+}
+.default-banner a { color: var(--bad); }
 #tree-filterbar small { opacity: 0.75; margin-left: 2px; }
 .tree-section .empty { padding: 24px; color: var(--muted); }
 .tree-section .clear { color: var(--bad); }

web/templates/tree_section.tmpl

@@ -1,5 +1,12 @@
 {{define "tree-section"}}
 <section id="tree-section" class="tree-section">
+  {{if .DefaultBanner}}
+    <p class="default-banner muted">
+      Showing default view: <strong>{{.DefaultBanner.Name}}</strong> ·
+      <a href="/?nodefault=1"
+         hx-get="/?nodefault=1" hx-target="#tree-section" hx-swap="outerHTML" hx-push-url="true">clear</a>
+    </p>
+  {{end}}
   <p class="counts">
     <strong>{{.Matched}}</strong> / <strong>{{.Total}}</strong> items match
     {{if .OrphanN}} · <strong>{{.OrphanN}}</strong> unclassified mai-managed roots <a href="/admin/classify">→ classify</a>{{end}}

web/views.go

@@ -210,6 +210,41 @@ func filterQueryToJSON(query string) ([]byte, error) {
 	return json.Marshal(payload)
 }
 
+// applyDefaultView resolves the saved view marked is_default_for=<page>
+// when the request URL carries no filter/view-specific params and the user
+// has not opted out via ?nodefault=1. Returns the applied view (for banner
+// labelling) or nil when no default exists / was applied.
+//
+// Per design.md §7 Slice E: defaults are a polish layer. They only kick in
+// on a "clean" landing — the moment the user types a chip click, the URL
+// gains a filter param and the default no longer auto-applies. Same with
+// an explicit ?view=<uuid>.
+func (s *Server) applyDefaultView(r *http.Request, page string, filter *TreeFilter, viewType *string) (*store.View, error) {
+	q := r.URL.Query()
+	if q.Get("nodefault") == "1" {
+		return nil, nil
+	}
+	// Any filter-affecting param means "user is driving" — skip the default.
+	for _, key := range []string{"q", "tag", "mgmt", "status", "has", "show-archived", "public", "project", "project_id", "project_descendants", "view", "view_type", "group_by"} {
+		if q.Get(key) != "" {
+			return nil, nil
+		}
+	}
+	v, err := s.Store.DefaultViewFor(r.Context(), page)
+	if err != nil || v == nil {
+		return v, err
+	}
+	payload := map[string]any{}
+	if len(v.FilterJSON) > 0 {
+		if err := json.Unmarshal(v.FilterJSON, &payload); err != nil {
+			return v, fmt.Errorf("decode default filter_json: %w", err)
+		}
+	}
+	*filter = filterFromJSONPayload(payload)
+	*viewType = v.ViewType
+	return v, nil
+}
+
 // applySavedView resolves a `?view=<uuid>` reference and folds the persisted
 // filter + view_type back into the supplied TreeFilter + view-type slot.
 // Called by every Views-supporting page handler at the top of their render

web/views_test.go

@@ -89,6 +89,60 @@ func TestViewsCRUDRoundTrip(t *testing.T) {
 	}
 }
 
+// TestDefaultViewAppliedOnCleanURL verifies the Slice E behaviour: when /
+// is requested with no chip params and a default view exists for the page,
+// the saved filter + view_type apply and a "Showing default view: …"
+// banner renders. Adding any chip param (?tag=…) bypasses the default.
+// ?nodefault=1 is the explicit opt-out.
+func TestDefaultViewAppliedOnCleanURL(t *testing.T) {
+	srv, pool := mustServer(t)
+	defer pool.Close()
+	h := srv.Routes()
+	ctx := context.Background()
+
+	stamp := strings.ReplaceAll(time.Now().UTC().Format("150405.000"), ".", "")
+	name := "p5i-E-default-" + stamp
+	defer pool.Exec(context.Background(),
+		`UPDATE projax.views SET deleted_at = now() WHERE name = $1 AND deleted_at IS NULL`, name)
+
+	if _, err := pool.Exec(ctx, `
+INSERT INTO projax.views (name, view_type, filter_json, is_default_for)
+VALUES ($1, 'card', $2::jsonb, 'tree')`,
+		name, []byte(`{"tags":["work"]}`)); err != nil {
+		t.Fatalf("seed default view: %v", err)
+	}
+
+	// Clean URL: default applies → card view + banner.
+	_, body := get(t, h, "/")
+	if !strings.Contains(body, `class="tree-card-grid"`) {
+		t.Error("clean / should auto-apply default view (card grid expected)")
+	}
+	if !strings.Contains(body, `default-banner`) {
+		t.Error("default-banner should render when a default applies")
+	}
+	if !strings.Contains(body, name) {
+		t.Error("banner should name the applied default view")
+	}
+
+	// Any chip param bypasses the default → list view (no banner).
+	_, withChip := get(t, h, "/?tag=dev")
+	if strings.Contains(withChip, `default-banner`) {
+		t.Error("default banner should disappear once user types a chip")
+	}
+	if !strings.Contains(withChip, `<ul class="forest">`) {
+		t.Error("?tag=dev should render the forest (default not applied)")
+	}
+
+	// Explicit opt-out via ?nodefault=1.
+	_, optOut := get(t, h, "/?nodefault=1")
+	if strings.Contains(optOut, `default-banner`) {
+		t.Error("?nodefault=1 should suppress the default banner")
+	}
+	if !strings.Contains(optOut, `<ul class="forest">`) {
+		t.Error("?nodefault=1 should render the forest (default suppressed)")
+	}
+}
+
 // TestSavedViewAppliedOnQueryParam verifies that opening / with ?view=<uuid>
 // re-applies the saved filter+view_type. We seed a view tagged work=patents
 // and assert the rendered tree has the right ProjectChip / chip-on state.