feat: improve XLSX preview + add code syntax highlighting

XLSX QoL:
- Custom table renderer (excelToTable.ts) with column letters,
  row numbers, right-aligned numbers, empty cell handling
- Monospace font, sticky headers + row nums, cell cursor
- Sheet tabs moved to bottom bar (like PPTX navigation)
- Unified styles between FileNav and FileItemModal

Code highlighting:
- Shiki-based syntax highlighting for code files in FileNav
- Line numbers, dark/light theme support
- Source/Preview toggle for code files

src/lib/utils/codeHighlight.ts

@@ -0,0 +1,95 @@
+import { codeToHtml, bundledLanguages } from 'shiki';
+
+/**
+ * Map file extensions to Shiki language identifiers.
+ * Only extensions whose Shiki lang id differs from the extension itself need explicit entries.
+ */
+const EXT_OVERRIDE: Record<string, string> = {
+	py: 'python',
+	js: 'javascript',
+	ts: 'typescript',
+	jsx: 'jsx',
+	tsx: 'tsx',
+	rb: 'ruby',
+	rs: 'rust',
+	kt: 'kotlin',
+	cs: 'csharp',
+	fs: 'fsharp',
+	sh: 'bash',
+	bash: 'bash',
+	zsh: 'bash',
+	yml: 'yaml',
+	md: 'markdown',
+	mdx: 'mdx',
+	dockerfile: 'dockerfile',
+	tf: 'terraform',
+	hcl: 'hcl',
+	ex: 'elixir',
+	exs: 'elixir',
+	erl: 'erlang',
+	hs: 'haskell',
+	ml: 'ocaml',
+	mli: 'ocaml',
+	pl: 'perl',
+	pm: 'perl',
+	r: 'r',
+	m: 'objective-c',
+	mm: 'objective-cpp',
+	h: 'c',
+	hpp: 'cpp',
+	cc: 'cpp',
+	cxx: 'cpp',
+	proto: 'proto',
+	nim: 'nim',
+	zig: 'zig',
+	v: 'v',
+	svelte: 'svelte',
+	vue: 'vue',
+	astro: 'astro',
+	prisma: 'prisma',
+	graphql: 'graphql',
+	gql: 'graphql',
+	jsonc: 'jsonc',
+	jsonl: 'jsonl'
+};
+
+const _langSet = new Set(Object.keys(bundledLanguages));
+
+/**
+ * Resolve a file extension to a Shiki language id, or null if not supported.
+ */
+export function extToLang(ext: string): string | null {
+	const lower = ext.toLowerCase();
+	// explicit override first
+	if (EXT_OVERRIDE[lower]) return EXT_OVERRIDE[lower];
+	// if the extension itself is a bundled language id (e.g. 'go', 'rust', 'sql', 'toml', ...)
+	if (_langSet.has(lower)) return lower;
+	return null;
+}
+
+/**
+ * Returns true if the given file path has a code-file extension that Shiki can highlight.
+ */
+export function isCodeFile(path: string | null): boolean {
+	if (!path) return false;
+	const ext = path.split('.').pop()?.toLowerCase() ?? '';
+	return extToLang(ext) !== null;
+}
+
+/**
+ * Highlight code using Shiki with dual light/dark themes via CSS variables.
+ * Returns an HTML string. Throws on failure.
+ */
+export async function highlightCode(code: string, filePath: string): Promise<string> {
+	const ext = filePath.split('.').pop()?.toLowerCase() ?? '';
+	const lang = extToLang(ext) ?? 'text';
+
+	return await codeToHtml(code, {
+		lang,
+		themes: {
+			light: 'github-light',
+			dark: 'github-dark'
+		},
+		defaultColor: 'light'
+	});
+}