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',

web/i18n/fr.js

@@ -954,6 +954,19 @@ export default {
   'fv.mode_source':         'Afficher la source',
   'fv.binary_unavailable':  'Aperçu non disponible pour ce type de fichier.',
   'fv.latex_failed':        'Échec de la compilation LaTeX — affichage de la source à la place',
+  'fv.tab_view':            'Afficher',
+  'fv.tab_edit':            'Modifier',
+  'fv.dirty_badge':         'Modifications non enregistrées',
+  'fv.dirty_warn':          'Abandonner les modifications non enregistrées ?',
+  'fv.save':                'Enregistrer',
+  'fv.saving':              'Enregistrement…',
+  'fv.cancel':              'Annuler',
+  'fv.edit_placeholder':    'Écrire en Markdown…',
+  'fv.edit_hint':           'Basculer vers Afficher pour l\'aperçu',
+  'fv.conflict_title':      'Le fichier a été modifié à distance pendant votre modification. Que voulez-vous faire ?',
+  'fv.conflict_reload':     'Recharger la version distante',
+  'fv.conflict_copy':       'Copier les miennes, puis recharger',
+  'fv.conflict_overwrite':  'Écraser',
 
   // ── Marketplace ─────────────────────────────────────────────────────────────
   'marketplace.title':               'Marketplace',

web/i18n/it.js

@@ -954,6 +954,19 @@ export default {
   'fv.mode_source':         'Mostra sorgente',
   'fv.binary_unavailable':  'Anteprima non disponibile per questo tipo di file.',
   'fv.latex_failed':        'Compilazione LaTeX fallita — mostra il sorgente',
+  'fv.tab_view':            'Visualizza',
+  'fv.tab_edit':            'Modifica',
+  'fv.dirty_badge':         'Modifiche non salvate',
+  'fv.dirty_warn':          'Scartare le modifiche non salvate?',
+  'fv.save':                'Salva',
+  'fv.saving':              'Salvataggio…',
+  'fv.cancel':              'Annulla',
+  'fv.edit_placeholder':    'Scrivi in Markdown…',
+  'fv.edit_hint':           'Passa a Visualizza per l\'anteprima',
+  'fv.conflict_title':      'Il file è stato modificato da remoto mentre stavi scrivendo. Cosa vuoi fare?',
+  'fv.conflict_reload':     'Ricarica remoto',
+  'fv.conflict_copy':       'Copia le mie, poi ricarica',
+  'fv.conflict_overwrite':  'Sovrascrivi',
 
   // ── Marketplace ──────────────────────────────────────────────────────────────
   'marketplace.title':               'Marketplace',