Add favicon link tags so Safari renders an icon

[disnet]

Summary

• Adds explicit <link rel="icon"> (SVG + PNG fallback), apple-touch-icon, mask-icon, and theme-color to frontend/src/app.html so Safari has a discoverable favicon on first paint.
• Previously app.html had zero icon tags; the only favicon link came from +layout.svelte's <svelte:head> and was SVG-only, which Safari ≤15 and iOS share/home-screen flows don't pick up.
• Reuses existing /icons/icon-512.{svg,png} assets — no new image files.

Notes

• The plan's "no other place injects favicon links" assumption was slightly off: frontend/src/routes/+layout.svelte:267 already declares <link rel="icon" type="image/svg+xml" href={Logo} />. That layout-injected tag still wins for the SVG icon (it renders after %sveltekit.head%), so the SVG line in app.html is a redundant baseline. The PNG, apple-touch-icon, and mask-icon entries are net-new and are what actually unblock Safari ≤15 and iOS.
• Safari aggressively caches favicons. Users who already loaded the old page may need a hard reload (Cmd+Opt+R) or to clear ~/Library/Safari/Favicon Cache/ before they see the icon.

Test plan

• npm run check — 0 errors (pre-existing warnings unchanged)
• npm run build — succeeds; build/icons/icon-512.{svg,png} emitted and the new link tags appear in build/index.html
• Open in Safari desktop, confirm tab favicon (hard-refresh)
• iOS Safari → Share → Add to Home Screen, confirm icon renders
• Chrome + Firefox: confirm no regression

Out of scope (follow-ups)

• Dedicated 180×180 apple-touch-icon.png and 32×32 favicon to avoid sending the 21 KB 512 PNG.
• static/favicon.ico to quiet bare /favicon.ico 404s.
• Monochrome SVG for mask-icon (current SVG is multi-color; Safari flattens it).

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