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/index.html

@@ -13,6 +13,7 @@
       <button class="tab active" data-view="log">Log</button>
       <button class="tab" data-view="history">History</button>
       <button class="tab" data-view="stats">Stats</button>
+      <button class="tab" data-view="settings">Settings</button>
     </nav>
 
     <!-- ═══ LOG VIEW ═══ -->
@@ -92,6 +93,19 @@
       </div>
     </div>
 
+    <!-- ═══ SETTINGS VIEW ═══ -->
+    <div id="view-settings" class="view">
+      <div class="card">
+        <label class="settings-row">
+          <div class="settings-row-label">
+            <div class="settings-row-title">Rest timer</div>
+            <div class="settings-row-hint">Show time since the last set was logged.</div>
+          </div>
+          <input type="checkbox" id="setting-rest-timer" class="settings-toggle" checked />
+        </label>
+      </div>
+    </div>
+
     <footer id="app-footer">
       <span id="version-badge"></span>
     </footer>