Add clear GPS endpoint + frontend button

- DELETE /api/assets/{id}/gps sets lat/lng to NULL
- Detail view shows 'Clear GPS' button when asset has GPS coords
- Confirmation modal before clearing
- Tests: 204 success + 404 not found

static/index.html

@@ -1249,6 +1249,7 @@
       </div>
       <div style="display:flex;gap:8px;margin-bottom:10px;">
         <button class="btn btn-outline btn-sm" onclick="editAsset()" style="flex:1;">✏️ Edit</button>
+        <button class="btn btn-outline btn-sm" id="detailClearGpsBtn" onclick="clearAssetGps()" style="flex:1;display:none;">📍 Clear GPS</button>
         <button class="btn btn-danger btn-sm" onclick="deleteAsset()" style="flex:1;">🗑️ Delete</button>
       </div>
     </div>
@@ -3605,6 +3606,14 @@
           navBtn.style.display = 'none';
         }
 
+        // Clear GPS button — show when asset has GPS
+        const clearGpsBtn = document.getElementById('detailClearGpsBtn');
+        if (a.latitude != null && a.longitude != null) {
+          clearGpsBtn.style.display = '';
+        } else {
+          clearGpsBtn.style.display = 'none';
+        }
+
         // Keys + Badges
         const keys = a.keys || [];
         const badges = a.badges || (Array.isArray(a.badge_names) ? a.badge_names : []);
@@ -3996,6 +4005,24 @@
       }
     }
 
+    // ── Clear GPS ──────────────────────────────────────────────────────────
+    async function clearAssetGps() {
+      if (!AppState.currentAssetId) return;
+      const confirmed = await showModal(
+        'Clear GPS Data',
+        'Remove GPS coordinates from this asset?',
+        'Clear GPS'
+      );
+      if (!confirmed) return;
+      try {
+        await api('/api/assets/' + AppState.currentAssetId + '/gps', { method: 'DELETE' });
+        showToast('GPS data cleared');
+        viewAsset(AppState.currentAssetId);
+      } catch (e) {
+        showToast(e.message, true);
+      }
+    }
+
     // ── Delete ────────────────────────────────────────────────────────────
     async function deleteAsset() {
       if (!AppState.currentAssetId) return;