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/zh-Hans/instance.ts

@@ -94,6 +94,19 @@ export const instanceMessages = {
   "instanceShell.rightPanel.tabs.files": "文件",
   "instanceShell.rightPanel.tabs.status": "状态",
   "instanceShell.rightPanel.tabs.ariaLabel": "右侧面板标签页",
+  "instanceShell.rightPanel.actions.refresh": "刷新",
+  "instanceShell.rightPanel.actions.save": "保存 (Ctrl+S)",
+  "instanceShell.rightPanel.actions.saveConfirm.message": "切换前是否保存对 \"{path}\" 的更改？",
+  "instanceShell.rightPanel.actions.saveConfirm.confirmLabel": "保存",
+  "instanceShell.rightPanel.actions.saveConfirm.cancelLabel": "放弃更改",
+  "instanceShell.rightPanel.actions.conflict.message": "文件已被代理修改。是否覆盖代理的更改？",
+  "instanceShell.rightPanel.actions.conflict.confirmLabel": "覆盖",
+  "instanceShell.rightPanel.actions.conflict.cancelLabel": "取消",
+  "instanceShell.rightPanel.actions.refreshDirty.message": "文件有未保存的更改。刷新将放弃您的编辑。继续？",
+  "instanceShell.rightPanel.actions.refreshDirty.confirmLabel": "刷新",
+  "instanceShell.rightPanel.actions.refreshDirty.cancelLabel": "取消",
+  "instanceShell.rightPanel.toast.saveSuccess": "文件保存成功",
+  "instanceShell.rightPanel.toast.saveError": "保存文件失败",
   "instanceShell.rightPanel.sections.sessionChanges": "会话更改",
   "instanceShell.rightPanel.sections.sessionChanges.tooltip": "当前会话中修改的文件。显示每个文件的添加和删除。",
   "instanceShell.rightPanel.sections.plan": "计划",