feat: update branding with official HiHala logo + 'Museums Data'

- Add official HiHala logo SVG (text portion from v1.5)
- Replace text-based nav brand with logo image + 'Museums Data'
- Update data source to default to NocoDB with Sheets fallback
- Move all credentials to environment variables (.env.local)

src/App.css

@@ -159,14 +159,22 @@ body {
 }
 
 .nav-brand {
-  font-size: 1.125rem;
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  font-size: 1rem;
   font-weight: 600;
-  color: var(--text-primary);
+  color: var(--text-secondary);
   letter-spacing: -0.02em;
 }
 
+.nav-logo {
+  height: 24px;
+  width: auto;
+}
+
 .nav-brand span {
-  color: var(--gold);
+  color: var(--text-secondary);
 }
 
 .nav-links {
@@ -1011,7 +1019,11 @@ table tbody tr:hover {
   }
   
   .nav-brand {
-    font-size: 1.1rem;
+    font-size: 0.9rem;
+  }
+  
+  .nav-logo {
+    height: 20px;
   }
   
   /* Mobile Bottom Navigation */