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

docs/projects.md

@@ -30,6 +30,13 @@ A file explorer rooted at the project folder:
 - Click a **folder** to navigate into it.
 - The listing **updates by itself**: if another member or the assistant creates, renames or deletes a file while you're looking at a folder, the change appears within a second — no refresh needed.
 
+For **Markdown** files (`.md`), if you have write access the viewer has two tabs:
+
+- **View** — the rendered document (the default).
+- **Edit** — edit the Markdown source directly. Switch back to View any time to preview your changes; **Save** writes the file, **Cancel** discards.
+
+Because the same file may be edited at the same time by another member, another of your tabs, or the assistant, saving is protected against silent overwrites: if the file changed on the server *after* you started editing, you'll see a banner — **Reload remote** (discard your edits and take the newer version), **Copy mine, then reload** (copy your edits to the clipboard, then take the remote version), or **Overwrite** (force your version). So no one's work is ever lost without you choosing.
+
 If you have write access you can also, from the toolbar or each row:
 
 - **New folder** — create a subfolder in the current location.