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/en/instance.ts

@@ -95,6 +95,18 @@ export const instanceMessages = {
   "instanceShell.rightPanel.tabs.status": "Status",
   "instanceShell.rightPanel.tabs.ariaLabel": "Right panel tabs",
   "instanceShell.rightPanel.actions.refresh": "Refresh",
+  "instanceShell.rightPanel.actions.save": "Save (Ctrl+S)",
+  "instanceShell.rightPanel.actions.saveConfirm.message": "Do you want to save changes to \"{path}\" before switching?",
+  "instanceShell.rightPanel.actions.saveConfirm.confirmLabel": "Save",
+  "instanceShell.rightPanel.actions.saveConfirm.cancelLabel": "Discard Changes",
+  "instanceShell.rightPanel.actions.conflict.message": "File was modified by the agent. Overwrite agent's changes?",
+  "instanceShell.rightPanel.actions.conflict.confirmLabel": "Overwrite",
+  "instanceShell.rightPanel.actions.conflict.cancelLabel": "Cancel",
+  "instanceShell.rightPanel.actions.refreshDirty.message": "File has unsaved changes. Refresh will discard your edits. Continue?",
+  "instanceShell.rightPanel.actions.refreshDirty.confirmLabel": "Refresh",
+  "instanceShell.rightPanel.actions.refreshDirty.cancelLabel": "Cancel",
+  "instanceShell.rightPanel.toast.saveSuccess": "File saved successfully",
+  "instanceShell.rightPanel.toast.saveError": "Failed to save file",
   "instanceShell.rightPanel.sections.sessionChanges": "Session Changes",
   "instanceShell.rightPanel.sections.sessionChanges.tooltip": "Files modified in the current session. Shows additions and deletions for each file.",
   "instanceShell.rightPanel.sections.plan": "Plan",