feat: profile/settings (rest timer toggle)

Settings infrastructure + one working preference:

- New user_settings table (JSON blob per user, so adding
  future keys needs no migration).
- db.get_settings / update_settings helpers (merge semantics).
- GET/PUT /api/settings endpoints.
- New Settings tab in the Mini App with a rest-timer on/off
  toggle. Setting is loaded on init and written through on
  change; the rest-timer display now respects it.

Units (kg/lb) and language are intentionally left unwired for
now — each needs end-to-end display/input changes and deserve
focused passes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

webapp/style.css

@@ -486,6 +486,37 @@ details[open] .raw-toggle::before {
   opacity: 1;
 }
 
+/* ── Settings view ───────────────────────────────────────────── */
+
+.settings-row {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 16px;
+  cursor: pointer;
+}
+
+.settings-row-label { flex: 1; }
+
+.settings-row-title {
+  font-size: 15px;
+  font-weight: 500;
+  color: var(--tg-theme-text-color, #000);
+}
+
+.settings-row-hint {
+  font-size: 12px;
+  color: var(--tg-theme-hint-color, #999);
+  margin-top: 2px;
+}
+
+.settings-toggle {
+  width: 20px;
+  height: 20px;
+  cursor: pointer;
+  accent-color: var(--tg-theme-button-color, #2481cc);
+}
+
 /* ── Footer / version badge ──────────────────────────────────── */
 
 #app-footer {