feat(glass): variable-drive the glass system + dev GlassTuner (Phase 0/0b)

Foundation for the app-wide glass-consistency epic:
- index.css: every look-driving value (blur/saturate/dark-brightness/panel-card-
  menu opacity/border/inset/edge/scrim/ambient + mosaic) is now a CSS custom
  property defaulting to the historical hard-coded value — one-place tunable, no
  visual change until a value is bumped.
- GlassTuner.tsx: dev-only (localhost-gated) always-on corner panel that live-
  drives those vars, with treatment presets (Adaptive/Gradient+/Edge+scrim),
  per-scheme palette editor, muted thumbnail-mosaic toggle, localStorage persist,
  and Copy-CSS export. Deleted once the tuned finals are baked in.
- MosaicBackdrop.tsx: opt-in muted (blur+darken+desaturate) mosaic of on-screen
  thumbnails so dark glass has real content to refract.

frontend/src/App.tsx

@@ -50,6 +50,7 @@ import Header from "./components/Header";
 import NavSidebar from "./components/NavSidebar";
 import Sidebar from "./components/Sidebar";
 import BackToTop from "./components/BackToTop";
+import GlassTuner from "./components/GlassTuner";
 import ChatDock from "./components/ChatDock";
 import Toaster from "./components/Toaster";
 import ErrorDialog from "./components/ErrorDialog";
@@ -903,6 +904,9 @@ export default function App() {
       <ErrorDialog />
       {/* Re-binds to the active page's scroll container on navigation (page or channel change). */}
       <BackToTop dep={channelView ? `chan:${channelView.id}` : page} />
+      {/* DEV-ONLY live appearance tuner (localhost-gated; self-returns null on prod). Delete when
+          the glass-consistency epic bakes the final values into index.css. */}
+      <GlassTuner />
     </div>
   );
 }