file viewer: edit Markdown with optimistic-lock conflict detection

- GET /api/file returns ETag (mtime+size) + X-Writable on disk files;
  PUT /api/file accepts optional if_match -> 409 Conflict on stale version
  (last-write-wins preserved when omitted), echoes the new ETag
- FileViewerBase: View | Edit tabs for .md when the caller can write;
  source textarea with Save/Cancel, live preview while editing
- Watcher no longer clobbers the buffer mid-edit: while editing with
  unsaved changes it probes the server ETag and only raises a conflict
  when the remote actually moved on (own-save echo is ignored)
- Conflict banner: Reload remote | Copy mine, then reload | Overwrite
- i18n (en/it/fr) + CSS; docs/projects.md updated

src/frontend/api/mod.rs

@@ -238,6 +238,13 @@ impl ApiError {
     pub fn payload_too_large(msg: impl Into<String>) -> Self {
         Self { status: StatusCode::PAYLOAD_TOO_LARGE, message: msg.into() }
     }
+
+    /// `409 Conflict` — used by the file editor's optimistic locking: the
+    /// caller sent an `if_match` ETag that no longer matches the file on disk
+    /// (someone else — another user, another tab, or an agent — wrote first).
+    pub fn conflict(msg: impl Into<String>) -> Self {
+        Self { status: StatusCode::CONFLICT, message: msg.into() }
+    }
 }
 
 /// Resolves the authenticated caller's per-user runtime context, or `401` when the