perf(boot): instant HTML splash in the shell for a faster perceived first paint

The app is a pure client-rendered SPA (ssr=false, adapter-static), so until the
JS bundle downloads, parses and mounts, the page is blank — only then does the
layout's own loading state appear. True route prerendering would mean turning on
SSR, an architectural change at odds with the SPA design.

Instead, bake a tiny boot splash into the shell (`app.html`): a centered spinner
that paints the moment the HTML is parsed — before the app bundle or its CSS
load — covering the blank gap. The root layout removes `#app-splash` the instant
it mounts (before `session.load`), so public routes like /login (which render
without waiting for the session) appear immediately and protected routes hand
off to their own loading UI. `light-dark()` plus early `color-scheme` rules make
the splash match the resolved theme (incl. the saved override), so there's no
colour flash when the app CSS arrives; it respects `prefers-reduced-motion`.

Pure HTML/CSS in the shell — no new requests, no JS framework on the critical
path, ~0.6 KB in index.html. Verified on the real static-dist build in headless
Chromium: the shell ships `#app-splash`, the layout removes it after mount, the
app renders, and there are no runtime errors.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M8Vb9QHmLZnEMzHz7MrFy6

frontend/src/routes/+layout.svelte

@@ -21,6 +21,11 @@
 	let ready = $state(false);
 
 	onMount(async () => {
+		// The instant HTML boot splash has done its job — the app is mounted, so
+		// the route (login renders immediately; protected routes show their own
+		// loading state) is already in the DOM behind it.
+		document.getElementById('app-splash')?.remove();
+
 		// Redirect old `#/...` bookmarks to the new path before anything else.
 		if (typeof location !== 'undefined' && location.hash.startsWith('#/')) {
 			const mapped = hashUrlToPath(location.hash);