ui: tokenise .fb-question spacing + dark-mode QA notes

Final polish + verification commit. Tokenises the last hard-coded
margin in .fb-question (1.25rem → var(--space-4)) so the spacing scale
introduced in commit 1 is the single source of truth. Visually
identical (1.25rem === --space-4); the payoff is that any future
adjustment to the field-gap token propagates here automatically.

Optimistic status toggle was implemented inline in the list (commit 4)
and detail (commit 5) pages with a revert-on-failure path — no further
sweep needed.

Dark-mode QA — verified at 375 / 1024 / 1440 widths via headless
Chromium with --enable-features=WebContentsForceDark + --force-dark-mode
(token cascade confirmed firing: the rendered gradient matches the dark
--gradient-bg in feedback.css, not Chrome's auto-dark inversion of the
light gradient):

- /  — wordmark + tagline + ghost CTA centred, dark-teal gradient ✓
- /login — vertical-centred form, white-on-dark fields, primary CTA ✓

Admin pages and /f/[slug] need post-deploy verification once head
merges + deploys this branch — they require auth + a real form slug
which the local preview can't supply. The token cascade is shared
with these pages so visual regressions are unlikely; functional QA
of the optimistic toggle, ⋯ menu click-outside, and share-strip
short-link flow should happen against fdbck.msbls.de.

src/lib/styles/feedback.css

@@ -238,7 +238,7 @@ body { min-height: 100vh; }
 }
 
 .fb-question {
-	margin-bottom: 1.25rem;
+	margin-bottom: var(--space-4);
 }
 
 .fb-question__label {