feat: Enable file editing and saving (#252)

## Summary
- Adds file writing capability to Monaco editor in the file viewer
- Implements writeFile API on the server for workspace files
- Integrates save functionality into the file viewer UI with proper
state management

## Bug Fixes (Review Feedback)
- Fixed failed save discarding edits when switching files - now checks
save result and only proceeds if successful
- Fixed refresh overwriting dirty editor state - now prompts for
confirmation before discarding edits
- Fixed save button unable to save empty files - changed check from `if
(content)` to `if (content !== undefined && content !== null)`
- Added agent edit conflict detection - when agent edits file while user
has unsaved changes, shows conflict dialog with Overwrite/Cancel options
- Fixed dialog appearing behind unpinned sidebar - increased alert
dialog z-index to z-100

## Related Issues
- Closes #251

---------

Co-authored-by: Jess Chadwick <jchadwick@gmail.com>

packages/ui/src/lib/i18n/messages/ja/instance.ts

@@ -94,6 +94,19 @@ export const instanceMessages = {
   "instanceShell.rightPanel.tabs.files": "ファイル",
   "instanceShell.rightPanel.tabs.status": "ステータス",
   "instanceShell.rightPanel.tabs.ariaLabel": "右パネルのタブ",
+  "instanceShell.rightPanel.actions.refresh": "更新",
+  "instanceShell.rightPanel.actions.save": "保存 (Ctrl+S)",
+  "instanceShell.rightPanel.actions.saveConfirm.message": "「{path}」への変更を切り替え前に保存しますか？",
+  "instanceShell.rightPanel.actions.saveConfirm.confirmLabel": "保存",
+  "instanceShell.rightPanel.actions.saveConfirm.cancelLabel": "変更を破棄",
+  "instanceShell.rightPanel.actions.conflict.message": "ファイルはエージェントによって変更されました。上書きしますか？",
+  "instanceShell.rightPanel.actions.conflict.confirmLabel": "上書き",
+  "instanceShell.rightPanel.actions.conflict.cancelLabel": "キャンセル",
+  "instanceShell.rightPanel.actions.refreshDirty.message": "ファイルには未保存の変更があります。更新すると編集が破棄されます。続行しますか？",
+  "instanceShell.rightPanel.actions.refreshDirty.confirmLabel": "更新",
+  "instanceShell.rightPanel.actions.refreshDirty.cancelLabel": "キャンセル",
+  "instanceShell.rightPanel.toast.saveSuccess": "ファイルを保存しました",
+  "instanceShell.rightPanel.toast.saveError": "ファイルの保存に失敗しました",
   "instanceShell.rightPanel.sections.sessionChanges": "セッション変更",
   "instanceShell.rightPanel.sections.sessionChanges.tooltip": "現在のセッションで変更されたファイル。各ファイルの追加と削除を表示します。",
   "instanceShell.rightPanel.sections.plan": "計画",