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/components/file-viewer/monaco-file-viewer.tsx

@@ -0,0 +1,89 @@
+import { createEffect, createSignal, onCleanup, onMount } from "solid-js"
+import { loadMonaco } from "../../lib/monaco/setup"
+import { getOrCreateTextModel } from "../../lib/monaco/model-cache"
+import { inferMonacoLanguageId } from "../../lib/monaco/language"
+import { ensureMonacoLanguageLoaded } from "../../lib/monaco/setup"
+import { useTheme } from "../../lib/theme"
+
+interface MonacoFileViewerProps {
+  scopeKey: string
+  path: string
+  content: string
+}
+
+export function MonacoFileViewer(props: MonacoFileViewerProps) {
+  const { isDark } = useTheme()
+  let host: HTMLDivElement | undefined
+
+  let editor: any = null
+  let monaco: any = null
+  const [ready, setReady] = createSignal(false)
+
+  const disposeEditor = () => {
+    try {
+      editor?.setModel(null)
+    } catch {
+      // ignore
+    }
+    try {
+      editor?.dispose()
+    } catch {
+      // ignore
+    }
+    editor = null
+  }
+
+  onMount(() => {
+    let cancelled = false
+    void (async () => {
+      monaco = await loadMonaco()
+      if (cancelled) return
+      if (!host || !monaco) return
+
+      monaco.editor.setTheme(isDark() ? "vs-dark" : "vs")
+      editor = monaco.editor.create(host, {
+        value: "",
+        language: "plaintext",
+        readOnly: true,
+        automaticLayout: true,
+        lineNumbers: "on",
+        minimap: { enabled: false },
+        scrollBeyondLastLine: false,
+        wordWrap: "off",
+        renderWhitespace: "selection",
+        fontSize: 13,
+      })
+
+      setReady(true)
+    })()
+
+    onCleanup(() => {
+      cancelled = true
+      setReady(false)
+      disposeEditor()
+    })
+  })
+
+  createEffect(() => {
+    if (!ready() || !monaco || !editor) return
+    monaco.editor.setTheme(isDark() ? "vs-dark" : "vs")
+  })
+
+  createEffect(() => {
+    if (!ready() || !monaco || !editor) return
+    const languageId = inferMonacoLanguageId(monaco, props.path)
+    const cacheKey = `${props.scopeKey}:file:${props.path}`
+    const model = getOrCreateTextModel({ monaco, cacheKey, value: props.content, languageId })
+    editor.setModel(model)
+
+    void ensureMonacoLanguageLoaded(languageId).then(() => {
+      try {
+        monaco.editor.setModelLanguage(model, languageId)
+      } catch {
+        // ignore
+      }
+    })
+  })
+
+  return <div class="monaco-viewer" ref={host} />
+}