refactor: rename 'Offline Mode' to 'Flight Mode'

- Change 'Local Only' to 'Flight Mode' in relay status indicator
- Rename settings section from 'Offline Mode' to 'Flight Mode'
- Better reflects the airplane icon metaphor
- More intuitive terminology for local-only relay mode

src/components/RelayStatusIndicator.tsx

@@ -57,7 +57,7 @@ export const RelayStatusIndicator: React.FC<RelayStatusIndicatorProps> = ({ rela
           </>
         ) : (
           <>
-            <span className="relay-status-title">Local Only</span>
+            <span className="relay-status-title">Flight Mode</span>
             <span className="relay-status-subtitle">{connectedUrls.length} local relay{connectedUrls.length !== 1 ? 's' : ''}</span>
           </>
         )}

src/components/Settings/OfflineModeSettings.tsx

@@ -18,7 +18,7 @@ const OfflineModeSettings: React.FC<OfflineModeSettingsProps> = ({ settings, onU
 
   return (
     <div className="settings-section">
-      <h3 className="section-title">Offline Mode</h3>
+      <h3 className="section-title">Flight Mode</h3>
       
       <div className="setting-group">
         <label htmlFor="useLocalRelayAsCache" className="checkbox-label">