feat(ui): double-click a port to start a cable draw

Saves the cable-tool arming step. Double-click sets
state.cableDrawFromPortID + clears selection, then the existing
cable-draw-in-progress branch in onPortPointerDown commits the cable
on the next port click. Esc / plain click still cancels.

Single-click behaviour (select + open port inspector) unchanged.

web/static/main.js

@@ -471,6 +471,17 @@ function renderCanvas() {
       });
       // Port-click drives both cable-draw (slice 7) and port-select (this fix).
       c.addEventListener("pointerdown", (e) => onPortPointerDown(e, prt));
+      // Double-click a port → enter cable-draw mode from that port,
+      // bypassing the cable-tool arming step. The next port-click
+      // commits the cable (existing cable-draw-in-progress branch in
+      // onPortPointerDown).
+      c.addEventListener("dblclick", (e) => {
+        e.stopPropagation();
+        e.preventDefault();
+        state.cableDrawFromPortID = prt.id;
+        state.selection = null;
+        render();
+      });
       g.append(c);
     }