feat(notes): render code files with Shikiji syntax highlighting

Non-markdown files opened as stacked notes are now highlighted using
the existing markdown-it-shikiji pipeline (4-backtick fence wrapping)
with a h1 filename heading. Edit controls are hidden for code files.
Adds alloy language grammar and a fileLanguage utility mapping
extensions to Shikiji language IDs.

src/utils/fileLanguage.ts

@@ -0,0 +1,31 @@
+const EXT_TO_LANG: Record<string, string> = {
+  sh: "bash",
+  bash: "bash",
+  js: "javascript",
+  mjs: "javascript",
+  cjs: "javascript",
+  ts: "typescript",
+  mts: "typescript",
+  cts: "typescript",
+  md: "markdown",
+  mdx: "markdown",
+  html: "html",
+  htm: "html",
+  css: "css",
+  scss: "css",
+  json: "json",
+  jsonc: "json",
+  als: "alloy"
+}
+
+const MARKDOWN_EXTS = new Set(["md", "mdx"])
+
+export function isMarkdownPath(path: string): boolean {
+  const ext = path.split(".").pop()?.toLowerCase() ?? ""
+  return MARKDOWN_EXTS.has(ext)
+}
+
+export function getFileLanguage(path: string): string | null {
+  const ext = path.split(".").pop()?.toLowerCase() ?? ""
+  return EXT_TO_LANG[ext] ?? null
+}