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/lib/api/files.ts

@@ -0,0 +1,124 @@
+import { auth } from '$lib/auth.svelte';
+import { type ApiResult } from '$lib/api/client';
+
+export type FileEntry = {
+	name: string;
+	path: string;
+	type: 'file' | 'directory' | 'symlink';
+	size: number;
+	mode: number;
+	permissions: string;
+	owner: string;
+	group: string;
+	modified_at: number;
+	symlink_target?: string | null;
+};
+
+export type ListDirResponse = {
+	entries: FileEntry[];
+};
+
+const MAX_READABLE_SIZE = 10 * 1024 * 1024; // 10 MB
+
+/**
+ * Whether a file can be previewed as text in the browser.
+ * Binary/unreadable extensions and files > 10 MB should be downloaded instead.
+ */
+const BINARY_EXTENSIONS = new Set([
+	'.png', '.jpg', '.jpeg', '.gif', '.bmp', '.ico', '.webp', '.avif', '.svg',
+	'.mp3', '.mp4', '.wav', '.ogg', '.flac', '.avi', '.mkv', '.mov', '.webm',
+	'.zip', '.tar', '.gz', '.bz2', '.xz', '.7z', '.rar', '.zst',
+	'.pdf', '.doc', '.docx', '.xls', '.xlsx', '.ppt', '.pptx',
+	'.exe', '.dll', '.so', '.dylib', '.bin', '.o', '.a', '.class', '.pyc',
+	'.woff', '.woff2', '.ttf', '.otf', '.eot',
+	'.db', '.sqlite', '.sqlite3',
+	'.iso', '.img', '.dmg',
+]);
+
+export function isBinaryFile(name: string): boolean {
+	const dot = name.lastIndexOf('.');
+	if (dot === -1) return false;
+	return BINARY_EXTENSIONS.has(name.slice(dot).toLowerCase());
+}
+
+export function isFileTooLarge(size: number): boolean {
+	return size > MAX_READABLE_SIZE;
+}
+
+export function formatFileSize(bytes: number): string {
+	if (bytes === 0) return '0 B';
+	const units = ['B', 'KB', 'MB', 'GB', 'TB'];
+	const i = Math.floor(Math.log(bytes) / Math.log(1024));
+	const val = bytes / Math.pow(1024, i);
+	return `${val < 10 ? val.toFixed(1) : Math.round(val)} ${units[i]}`;
+}
+
+export async function listDir(sandboxId: string, path: string, depth = 1): Promise<ApiResult<ListDirResponse>> {
+	try {
+		const headers: Record<string, string> = { 'Content-Type': 'application/json' };
+		if (auth.token) headers['Authorization'] = `Bearer ${auth.token}`;
+
+		const res = await fetch(`/api/v1/sandboxes/${sandboxId}/files/list`, {
+			method: 'POST',
+			headers,
+			body: JSON.stringify({ path, depth }),
+		});
+
+		const data = await res.json();
+		if (!res.ok) return { ok: false, error: data?.error?.message ?? 'Failed to list directory' };
+		return { ok: true, data: data as ListDirResponse };
+	} catch {
+		return { ok: false, error: 'Unable to connect to the server' };
+	}
+}
+
+export async function readFile(sandboxId: string, path: string): Promise<ApiResult<string>> {
+	try {
+		const headers: Record<string, string> = { 'Content-Type': 'application/json' };
+		if (auth.token) headers['Authorization'] = `Bearer ${auth.token}`;
+
+		const res = await fetch(`/api/v1/sandboxes/${sandboxId}/files/read`, {
+			method: 'POST',
+			headers,
+			body: JSON.stringify({ path }),
+		});
+
+		if (!res.ok) {
+			try {
+				const data = await res.json();
+				return { ok: false, error: data?.error?.message ?? 'Failed to read file' };
+			} catch {
+				return { ok: false, error: `HTTP ${res.status}` };
+			}
+		}
+
+		const blob = await res.blob();
+		const text = await blob.text();
+		return { ok: true, data: text };
+	} catch {
+		return { ok: false, error: 'Unable to connect to the server' };
+	}
+}
+
+export async function downloadFile(sandboxId: string, path: string, filename: string): Promise<void> {
+	const headers: Record<string, string> = { 'Content-Type': 'application/json' };
+	if (auth.token) headers['Authorization'] = `Bearer ${auth.token}`;
+
+	const res = await fetch(`/api/v1/sandboxes/${sandboxId}/files/read`, {
+		method: 'POST',
+		headers,
+		body: JSON.stringify({ path }),
+	});
+
+	if (!res.ok) throw new Error('Download failed');
+
+	const blob = await res.blob();
+	const url = URL.createObjectURL(blob);
+	const a = document.createElement('a');
+	a.href = url;
+	a.download = filename;
+	document.body.appendChild(a);
+	a.click();
+	a.remove();
+	URL.revokeObjectURL(url);
+}