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/fr/instance.ts

@@ -94,6 +94,19 @@ export const instanceMessages = {
   "instanceShell.rightPanel.tabs.files": "Fichiers",
   "instanceShell.rightPanel.tabs.status": "Statut",
   "instanceShell.rightPanel.tabs.ariaLabel": "Onglets du panneau droit",
+  "instanceShell.rightPanel.actions.refresh": "Actualiser",
+  "instanceShell.rightPanel.actions.save": "Enregistrer (Ctrl+S)",
+  "instanceShell.rightPanel.actions.saveConfirm.message": "Voulez-vous enregistrer les modifications de \"{path}\" avant de changer ?",
+  "instanceShell.rightPanel.actions.saveConfirm.confirmLabel": "Enregistrer",
+  "instanceShell.rightPanel.actions.saveConfirm.cancelLabel": "Annuler les modifications",
+  "instanceShell.rightPanel.actions.conflict.message": "Le fichier a été modifié par l'agent. Écraser les modifications de l'agent ?",
+  "instanceShell.rightPanel.actions.conflict.confirmLabel": "Écraser",
+  "instanceShell.rightPanel.actions.conflict.cancelLabel": "Annuler",
+  "instanceShell.rightPanel.actions.refreshDirty.message": "Le fichier a des modifications non enregistrées. Actualiser supprimera vos modifications. Continuer ?",
+  "instanceShell.rightPanel.actions.refreshDirty.confirmLabel": "Actualiser",
+  "instanceShell.rightPanel.actions.refreshDirty.cancelLabel": "Annuler",
+  "instanceShell.rightPanel.toast.saveSuccess": "Fichier enregistré avec succès",
+  "instanceShell.rightPanel.toast.saveError": "Échec de l'enregistrement du fichier",
   "instanceShell.rightPanel.sections.sessionChanges": "Changements de session",
   "instanceShell.rightPanel.sections.sessionChanges.tooltip": "Fichiers modifiés dans la session actuelle. Affiche les ajouts et suppressions pour chaque fichier.",
   "instanceShell.rightPanel.sections.plan": "Plan",