Add syntax highlighting to file browser, harden capsules list

File browser:
- Add shiki-based syntax highlighting (lazy-loaded, zero initial bundle
  impact) with support for 30+ languages
- Cap highlighting at 2000 lines to avoid freezing on large files
- Pre-compute preview lines as derived state instead of re-splitting
  on every render
- Add content-visibility: auto on code lines for off-screen skip
- Remove per-line CSS transitions (unnecessary paint on 5000 elements)
- Cap row entrance animations to first 30 entries

Capsules list:
- Pause auto-refresh polling when browser tab is hidden
- Add empty state for search with no results
- Fix error state not clearing on successful refresh
- Fix action menu positioning near viewport edges
- Disable create button when no template selected

frontend/src/lib/components/CreateCapsuleDialog.svelte

@@ -1,5 +1,4 @@
 <script lang="ts">
-	import { onMount } from 'svelte';
 	import { createCapsule, listSnapshots, type Capsule, type CreateCapsuleParams, type Snapshot } from '$lib/api/capsules';
 
 	type Props = {
@@ -292,7 +291,7 @@
 				</button>
 				<button
 					onclick={handleCreate}
-					disabled={creating}
+					disabled={creating || !templateQuery.trim()}
 					class="flex items-center gap-2 rounded-[var(--radius-button)] bg-[var(--color-accent)] px-5 py-2 text-ui font-semibold text-white transition-all duration-150 hover:brightness-115 hover:-translate-y-px active:translate-y-0 disabled:opacity-50 disabled:hover:translate-y-0"
 				>
 					{#if creating}