feat(maps): add Stadia dark + tactical tile providers with API key support

- Add offline.stadia_key to OFFLINE_DEFAULTS in routes/offline.py
- Add stadia_dark and tactical tile providers to Settings.tileProviders
- Update getTileConfig() to inject Stadia API key or fall back to CartoDB dark
- Add setStadiaKey() method for saving and applying the API key
- Show/hide Stadia key row in setTileProvider() and _updateUI()
- Add Stadia options to tile provider select in settings modal
- Add Stadia API key input row to settings modal
- Add TDD tests for stadia_key backend

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

templates/partials/settings-modal.html

@@ -77,6 +77,8 @@
                         <option value="openstreetmap">OpenStreetMap</option>
                         <option value="cartodb_light">CartoDB Positron</option>
                         <option value="esri_world">ESRI World Imagery</option>
+                        <option value="stadia_dark">Stadia Alidade Dark</option>
+                        <option value="tactical">Stadia Tactical (minimal)</option>
                         <option value="custom">Custom URL</option>
                     </select>
                 </div>
@@ -90,6 +92,15 @@
                                onchange="Settings.setCustomTileUrl(this.value)">
                     </div>
                 </div>
+                <div class="settings-row" id="stadiaKeyRow" style="display: none;">
+                    <div class="settings-label" style="width: 100%;">
+                        <span class="settings-label-text">Stadia API Key</span>
+                        <span class="settings-label-desc">Free at <a href="https://client.stadiamaps.com/signup/" target="_blank" style="color: var(--accent-cyan);">stadiamaps.com</a></span>
+                        <input type="text" id="stadiaKeyInput" class="settings-input"
+                               placeholder="your-api-key-here"
+                               onchange="Settings.setStadiaKey(this.value)">
+                    </div>
+                </div>
             </div>
 
             <div class="settings-group">