fix(frontend): restore SSR favicon rendering on public pages

[jesposito]

Summary

Fixes #432 — favicon missing on public pages (still shown on admin backend).

Root cause: commit 8b01708 ("feat: eliminate FOUC with SSR accent color, plan config, and palette generation", Mar 28 2026) replaced the direct data-init of faviconUrl with a null + $effect pattern:

-// Initialize from server-loaded data for correct SSR
-let faviconUrl = $state<string | null>(data.faviconUrl);
+let faviconUrl = $state<string | null>(null);
+
+$effect(() => {
+    faviconUrl = data.faviconUrl;
+});

$effect never runs during SSR, so the server-rendered HTML always contained the fallback <link rel="icon" href="/favicon.png"> even when a custom favicon was configured. The regression was hidden on admin routes because admin hydrates and navigates as an SPA, so the $effect fires after hydration and patches svelte:head client-side. Public pages are hit more often as fresh SSR loads (direct links, search, social), where the fallback was visible.

Fix

Restore $state(data.faviconUrl) so the initial value is picked up during SSR. $state remains writable, so the existing onMount settings-load (line 274) and handleFaviconChange event handler (line 355) keep working unchanged.

This restores the intent of PR #345 (commit e0a123b), whose comment // Initialize from server-loaded data for correct SSR was preserved through the regression while the line it described was silently gutted.

Test plan

• cd frontend && npm run check — svelte-check passes (verified, no errors in +layout.svelte)
• Run make dev, upload a custom favicon via /admin/settings, hard-reload / and view page source — SSR HTML contains <link rel="icon" type="image/x-icon" href="/api/favicon">, not the /favicon.png fallback
• Navigate client-side between / and /admin — favicon remains correct (writable $state still updated by onMount/event handler)
• Clear the custom favicon in admin settings and hard-reload / — falls back to /favicon.png as expected

Fixes #432