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

web/i18n/en.js

@@ -964,6 +964,19 @@ export default {
   'fv.mode_source':         'Show source',
   'fv.binary_unavailable':  'Preview not available for this file type.',
   'fv.latex_failed':        'LaTeX compilation failed — showing source instead',
+  'fv.tab_view':            'View',
+  'fv.tab_edit':            'Edit',
+  'fv.dirty_badge':         'Unsaved changes',
+  'fv.dirty_warn':          'Discard unsaved changes?',
+  'fv.save':                'Save',
+  'fv.saving':              'Saving…',
+  'fv.cancel':              'Cancel',
+  'fv.edit_placeholder':    'Write Markdown…',
+  'fv.edit_hint':           'Switch to View to preview',
+  'fv.conflict_title':      'The file was modified remotely while you were editing. What do you want to do?',
+  'fv.conflict_reload':     'Reload remote',
+  'fv.conflict_copy':       'Copy mine, then reload',
+  'fv.conflict_overwrite':  'Overwrite',
 
   // ── Marketplace ─────────────────────────────────────────────────────────────
   'marketplace.title':               'Marketplace',