feat(theme): R10 S1 — semantic status colors (danger/warn/success)

Add theme-scoped --danger/--warn/--success (+ -fg) tokens wired into the v4
@theme inline, replacing ~110 raw red/amber/emerald/green palette refs across
34 files. Fixes light-theme contrast failures (fixed Tailwind shades fell to
~1.7-3:1 on white). Status colors are scheme-independent — recognised by hue,
shifting only by theme to hold contrast (bright on dark bg, darkened on light).
Solid fills use the paired -fg text; @source inline guarantees the full -fg set
exists. IMDb rating gold kept as a deliberate decorative non-status exception.

frontend/src/components/Welcome.tsx

@@ -498,7 +498,7 @@ function AuthCard() {
                 className={inputCls}
               />
             )}
-            {error && <p className="text-xs text-red-400">{error}</p>}
+            {error && <p className="text-xs text-danger">{error}</p>}
             <button
               type="submit"
               disabled={busy}
@@ -593,7 +593,7 @@ function AuthCard() {
                       {demoBusy ? t("welcome.auth.working") : t("welcome.auth.demoEnter")}
                     </button>
                   </div>
-                  {demoError && <p className="text-xs text-red-400">{demoError}</p>}
+                  {demoError && <p className="text-xs text-danger">{demoError}</p>}
                 </form>
               )}
             </>
@@ -605,7 +605,7 @@ function AuthCard() {
 }
 
 function Banner({ tone, children }: { tone: "ok" | "warn"; children: React.ReactNode }) {
-  const cls = tone === "ok" ? "bg-accent/15 text-fg" : "bg-amber-500/15 text-fg";
+  const cls = tone === "ok" ? "bg-accent/15 text-fg" : "bg-warn/15 text-fg";
   return (
     <div className={`rounded-xl px-3 py-2 text-xs leading-relaxed mt-3 ${cls}`}>{children}</div>
   );