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>

internal/web/server.go

@@ -285,7 +285,11 @@ func writeJSON(w http.ResponseWriter, v any) {
 	json.NewEncoder(w).Encode(v)
 }
 
+// fail answers with an error. Messages are capitalized here, once, so they read as sentences.
 func fail(w http.ResponseWriter, code int, msg string) {
+	if r := []rune(msg); len(r) > 0 {
+		msg = strings.ToUpper(string(r[:1])) + string(r[1:])
+	}
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(code)
 	json.NewEncoder(w).Encode(map[string]string{"error": msg})