perf(app): remove unused Google Fonts requests, add pre-JS loading spinner

- app.html: comment out 3 Google Fonts <link> tags (Noto Sans, Noto Serif,
  Roboto Mono) — no theme or component applies these families; all themes use
  system-ui. Saves 3 blocking network requests on every page load.
- app.html: add subtle CSS-only #ae_loader spinner (1.75rem ring, pointer-events:none)
  that shows during JS download + root load function, before Svelte mounts.
- +layout.svelte: add onMount to remove #ae_loader as soon as Svelte bootstraps;
  the existing is_hydrating frosted-glass overlay takes over from there.
- app.css: comment out orphaned Quicksand @font-face — font-family was never
  applied to any element so browser never fetched it anyway.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app.css

@@ -233,12 +233,14 @@ html.trusted_access #appShell {
     font-display: swap;
 } */
 
-/* modern theme */
-@font-face {
+/* modern theme — @font-face commented out 2026-05-19: Quicksand is declared but no
+   CSS rule applies font-family:'Quicksand' to any element, so the browser never
+   fetches this file. Re-enable if a theme or component starts using it. */
+/* @font-face {
     font-family: 'Quicksand';
     src: url('/fonts/Quicksand.ttf');
     font-display: swap;
-}
+} */
 
 /* :root [data-theme='modern'] { */
 /* --theme-rounded-base: 20px;