Add Files tab to capsule detail page with file browser and preview

Implements a split-panel file browser: directory tree on the left with
path input and breadcrumb navigation, file preview on the right with
line numbers. Binary/large files (>10MB) show a download prompt instead.

Also adds CopyButton component across capsule, snapshot, and template
pages, and fixes pre-existing type errors in StatsPanel and admin
templates page.

frontend/src/routes/dashboard/capsules/+page.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 	import CreateCapsuleDialog from '$lib/components/CreateCapsuleDialog.svelte';
+	import CopyButton from '$lib/components/CopyButton.svelte';
 	import { capsuleRunningCount } from '$lib/capsule-store.svelte';
 	import { onMount } from 'svelte';
 	import { toast } from '$lib/toast.svelte';
@@ -457,6 +458,7 @@
 						{:else}
 							<a href="/dashboard/capsules/{capsule.id}" class="font-mono text-ui text-[var(--color-text-bright)] hover:text-[var(--color-accent-bright)] transition-colors duration-150">{capsule.id}</a>
 						{/if}
+						<CopyButton value={capsule.id} />
 					</div>
 
 					<!-- Template -->