Restyle the dashboard to the Lightwell design language

Same tokens, sidebar shell, components and voice as Hammerhead: teal accent,
warm neutrals, status badges with text, dialogs with focus trapping that name
what they affect, empty states, skeleton loading, outline icons, 44px touch
targets and a slide-in drawer on phones. The strict CSP stays: no inline
styles or scripts, so icons and sizes use classes.

Adds docs/design.md and scripts/lint-design.mjs (run in CI), capitalizes
server error messages in one place, and rewrites interface copy to the voice
rules (sentence case, buttons that name the result, no dashes).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -24,6 +24,9 @@ jobs:
             exit 1
           fi
 
+      - name: Design rules
+        run: node scripts/lint-design.mjs
+
       - name: Vet
         run: go vet ./...