Add TFR warning banner for airspace restrictions

Fetches active Temporary Flight Restrictions from the FAA website,
filters by configured state (LOCATION_STATE env var), and displays
a red warning banner at the top of the dashboard when TFRs are present.
Data is cached for 30 minutes and degrades gracefully if the FAA is unreachable.

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

backend/internal/config/config.go

@@ -20,6 +20,9 @@ type Config struct {
 	LocationLon  float64 `envconfig:"LOCATION_LON" default:"-122.4194"`
 	LocationName string  `envconfig:"LOCATION_NAME" default:"San Francisco"`
 
+	// Location state (for filtering TFRs)
+	LocationState string `envconfig:"LOCATION_STATE" default:"CA"`
+
 	// Timezone configuration
 	Timezone string `envconfig:"TIMEZONE" default:"America/Los_Angeles"`