feat(ui): add Monaco changes/files right drawer viewers

Use OpenCode v2 file APIs for browsing and Monaco DiffEditor for session snapshot diffs, with local baseline language metadata and optional CDN language loading.

packages/ui/src/lib/monaco/language.ts

@@ -0,0 +1,70 @@
+type MonacoApi = any
+
+let cachedLanguageMaps:
+  | {
+      fileNameToId: Map<string, string>
+      extToId: Map<string, string>
+    }
+  | null = null
+
+function buildLanguageMaps(monaco: MonacoApi) {
+  if (cachedLanguageMaps) return cachedLanguageMaps
+
+  const fileNameToId = new Map<string, string>()
+  const extToId = new Map<string, string>()
+
+  const languages = typeof monaco?.languages?.getLanguages === "function" ? monaco.languages.getLanguages() : []
+  if (Array.isArray(languages)) {
+    for (const lang of languages) {
+      const id = typeof lang?.id === "string" ? lang.id : null
+      if (!id) continue
+
+      const filenames = Array.isArray(lang?.filenames) ? lang.filenames : []
+      for (const name of filenames) {
+        if (typeof name !== "string") continue
+        if (!fileNameToId.has(name)) fileNameToId.set(name, id)
+      }
+
+      const extensions = Array.isArray(lang?.extensions) ? lang.extensions : []
+      for (const ext of extensions) {
+        if (typeof ext !== "string") continue
+        // Monaco uses leading dots for extensions (e.g. ".ts").
+        if (!extToId.has(ext)) extToId.set(ext, id)
+      }
+    }
+  }
+
+  cachedLanguageMaps = { fileNameToId, extToId }
+  return cachedLanguageMaps
+}
+
+function overrideLanguageId(fileName: string): string | null {
+  // Git-style ignore/config files: treat as shell-like.
+  if (fileName === ".gitignore" || fileName === ".gitattributes" || fileName === ".gitmodules") return "shell"
+
+  // Monaco doesn't ship a dedicated Makefile tokenizer in our baseline.
+  if (fileName === "Makefile" || fileName.startsWith("Makefile.")) return "shell"
+
+  return null
+}
+
+export function inferMonacoLanguageId(monaco: MonacoApi, path: string | undefined | null): string {
+  const raw = String(path || "").trim()
+  const fileName = raw.split("/").pop() || raw
+
+  const override = overrideLanguageId(fileName)
+  if (override) return override
+
+  const maps = buildLanguageMaps(monaco)
+  const byName = maps.fileNameToId.get(fileName)
+  if (byName) return byName
+
+  const dot = fileName.lastIndexOf(".")
+  if (dot > 0) {
+    const ext = fileName.slice(dot)
+    const byExt = maps.extToId.get(ext)
+    if (byExt) return byExt
+  }
+
+  return "plaintext"
+}