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/api-client.ts

@@ -234,6 +234,16 @@ export const serverApi = {
       `/api/workspaces/${encodeURIComponent(id)}/files/content?${params.toString()}`,
     )
   },
+  writeWorkspaceFile(id: string, relativePath: string, contents: string): Promise<void> {
+    const params = new URLSearchParams({ path: relativePath })
+    return request(
+      `/api/workspaces/${encodeURIComponent(id)}/files/content?${params.toString()}`,
+      {
+        method: "PUT",
+        body: JSON.stringify({ contents }),
+      },
+    )
+  },
 
   fetchConfigOwner<T extends Record<string, any> = Record<string, any>>(owner: string): Promise<T> {
     return request<T>(`/api/storage/config/${encodeURIComponent(owner)}`)