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/css/file-viewer.css

@@ -53,6 +53,124 @@
 
 /* ── Markdown preview ────────────────────────────────────────────────────────── */
 
+.fv-md-wrap {
+  display: flex;
+  flex-direction: column;
+  min-height: 100%;
+}
+
+/* View | Edit tab bar — shown only when the caller can write the file. */
+.fv-md-tabs {
+  display: flex;
+  align-items: center;
+  gap: 0.15rem;
+  padding: 0.4rem 1rem 0;
+  border-bottom: 1px solid var(--bs-border-color);
+  flex-shrink: 0;
+}
+.fv-md-tab {
+  appearance: none;
+  background: none;
+  border: none;
+  border-bottom: 2px solid transparent;
+  padding: 0.35rem 0.85rem;
+  margin-bottom: -1px;
+  font-size: 0.85rem;
+  color: var(--bs-secondary-color);
+  cursor: pointer;
+  border-radius: 4px 4px 0 0;
+}
+.fv-md-tab:hover { color: var(--bs-body-color); }
+.fv-md-tab.active {
+  color: var(--bs-body-color);
+  border-bottom-color: var(--accent);
+  font-weight: 600;
+}
+.fv-md-dirty-dot {
+  width: 7px;
+  height: 7px;
+  border-radius: 50%;
+  background: var(--accent);
+  margin-left: 0.35rem;
+  flex-shrink: 0;
+}
+
+/* ── Source editor (Edit tab) ───────────────────────────────────────────────── */
+
+.fv-edit {
+  flex: 1 1 auto;
+  display: flex;
+  flex-direction: column;
+  min-height: 0;
+  padding: 0.75rem 1rem 1rem;
+  gap: 0.5rem;
+}
+.fv-edit-textarea {
+  flex: 1 1 auto;
+  min-height: 50vh;
+  width: 100%;
+  resize: vertical;
+  padding: 0.85rem 1rem;
+  font-family: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, monospace;
+  font-size: 0.85rem;
+  line-height: 1.55;
+  tab-size: 4;
+  color: var(--bs-body-color);
+  background: var(--bs-secondary-bg);
+  border: 1px solid var(--bs-border-color);
+  border-radius: 6px;
+  white-space: pre;
+  overflow-wrap: normal;
+  overflow: auto;
+}
+.fv-edit-textarea:focus {
+  outline: none;
+  border-color: var(--accent);
+  box-shadow: 0 0 0 2px color-mix(in srgb, var(--accent) 30%, transparent);
+}
+.fv-edit-toolbar {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  flex-shrink: 0;
+}
+.fv-edit-hint {
+  font-size: 0.78rem;
+  color: var(--bs-secondary-color);
+  margin-left: auto;
+}
+
+/* ── Conflict banner (remote changed while editing) ─────────────────────────── */
+
+.fv-conflict-banner {
+  display: flex;
+  flex-wrap: wrap;
+  align-items: center;
+  gap: 0.6rem;
+  padding: 0.6rem 0.85rem;
+  background: var(--bs-warning-bg-subtle, #fff3cd);
+  border: 1px solid var(--bs-warning-border-subtle, #ffe69c);
+  color: var(--bs-emphasis-color, inherit);
+  border-radius: 6px;
+  flex-shrink: 0;
+}
+.fv-conflict-banner > .bi {
+  color: var(--bs-warning-text-emphasis, #664d03);
+  font-size: 1.05rem;
+  flex-shrink: 0;
+}
+.fv-conflict-text {
+  flex: 1 1 200px;
+  font-size: 0.82rem;
+  min-width: 0;
+}
+.fv-conflict-actions {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 0.4rem;
+  flex-shrink: 0;
+}
+
 .fv-md {
   max-width: 100%;
   padding: 1.5rem 2rem;