feat: settings modal with database export and import

Adds a gear button to the nav that opens a settings modal with:
- Export: GET /api/export returns all instances as a JSON backup file
  with a Content-Disposition attachment header
- Import: POST /api/import validates and bulk-replaces all instances;
  client uses FileReader to POST the parsed JSON, with a confirm dialog
  before destructive replace

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

css/app.css

@@ -70,6 +70,19 @@ nav {
 
 .nav-sep { flex: 1; }
 
+.nav-btn {
+  background: none;
+  border: 1px solid var(--border2);
+  color: var(--text2);
+  border-radius: 6px;
+  padding: 4px 8px;
+  font-size: 14px;
+  cursor: pointer;
+  margin-left: 10px;
+  line-height: 1;
+}
+.nav-btn:hover { border-color: var(--accent); color: var(--accent); }
+
 .nav-divider { color: var(--border2); }
 
 .nav-status {
@@ -615,6 +628,31 @@ select:focus { border-color: var(--accent); }
 
 .confirm-actions { display: flex; justify-content: flex-end; gap: 10px; }
 
+/* ── SETTINGS MODAL ── */
+.settings-section { padding: 16px 0; border-bottom: 1px solid var(--border); }
+.settings-section:last-child { border-bottom: none; padding-bottom: 0; }
+.settings-section-title {
+  font-size: 10px;
+  font-weight: 600;
+  text-transform: uppercase;
+  letter-spacing: 0.1em;
+  color: var(--text3);
+  margin-bottom: 8px;
+}
+.settings-desc { font-size: 12px; color: var(--text2); margin: 0 0 14px; line-height: 1.6; }
+.import-row { display: flex; gap: 10px; align-items: center; }
+.import-file-input { flex: 1; }
+
+.btn-secondary {
+  background: var(--bg3);
+  border-color: var(--border2);
+  color: var(--text);
+}
+.btn-secondary:hover { border-color: var(--accent); color: var(--accent); }
+
+.btn-danger { background: var(--red2); border-color: var(--red); color: var(--text); }
+.btn-danger:hover { background: var(--red); }
+
 /* ── SCROLLBAR ── */
 ::-webkit-scrollbar       { width: 6px; }
 ::-webkit-scrollbar-track { background: var(--bg); }