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/model-cache.ts

@@ -0,0 +1,53 @@
+type MonacoApi = any
+
+type CachedModel = {
+  model: any
+}
+
+const MAX_MODELS = 5
+
+// LRU map: newest at the end.
+const models = new Map<string, CachedModel>()
+
+function touch(key: string, entry: CachedModel) {
+  models.delete(key)
+  models.set(key, entry)
+}
+
+function evictIfNeeded() {
+  while (models.size > MAX_MODELS) {
+    const oldestKey = models.keys().next().value as string | undefined
+    if (!oldestKey) return
+    const entry = models.get(oldestKey)
+    models.delete(oldestKey)
+    try {
+      entry?.model.dispose()
+    } catch {
+      // ignore
+    }
+  }
+}
+
+export function getOrCreateTextModel(params: {
+  monaco: MonacoApi
+  cacheKey: string
+  value: string
+  languageId: string
+}): any {
+  const existing = models.get(params.cacheKey)
+  if (existing) {
+    touch(params.cacheKey, existing)
+    if (existing.model.getValue() !== params.value) {
+      existing.model.setValue(params.value)
+    }
+    return existing.model
+  }
+
+  const uri = params.monaco.Uri.parse(`opencode://model/${encodeURIComponent(params.cacheKey)}`)
+  // Create as plaintext. We'll set the final language after its contribution is loaded.
+  const model = params.monaco.editor.createModel(params.value, "plaintext", uri)
+  const entry = { model }
+  models.set(params.cacheKey, entry)
+  evictIfNeeded()
+  return model
+}