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/components/instance/instance-shell2.tsx

@@ -420,6 +420,7 @@ const InstanceShell2: Component<InstanceShellProps> = (props) => {
         onClose={closeLeftDrawer}
         ModalProps={modalProps}
         sx={{
+          zIndex: 60,
           "& .MuiDrawer-paper": {
             width: isPhoneLayout() ? "100vw" : `${sessionSidebarWidth()}px`,
             boxSizing: "border-box",
@@ -530,6 +531,7 @@ const InstanceShell2: Component<InstanceShellProps> = (props) => {
         onClose={closeRightDrawer}
         ModalProps={modalProps}
         sx={{
+          zIndex: 60,
           "& .MuiDrawer-paper": {
             width: isPhoneLayout() ? "100vw" : `${rightDrawerWidth()}px`,
             boxSizing: "border-box",