feat: temperature unit preference and slot timezone display for briefing

- Add °C/°F toggle in briefing settings; persisted in briefing_config.temp_unit
- briefing_pipeline reads temp_unit and converts Open-Meteo Celsius values
  before passing them to the LLM (both full compilation and slot injection)
- Scheduled Slots section now shows each UTC slot time converted to the
  user's browser local time, plus a line confirming which timezone the
  browser is using

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

frontend/src/api/client.ts

@@ -328,6 +328,7 @@ export interface BriefingConfig {
   work_days: number[];
   slots: BriefingSlots;
   notifications: boolean;
+  temp_unit: 'C' | 'F';
 }
 
 export interface BriefingFeed {
@@ -360,6 +361,7 @@ const DEFAULT_BRIEFING_CONFIG: BriefingConfig = {
   work_days: [1, 2, 3, 4, 5],
   slots: { compilation: true, morning: true, midday: false, afternoon: false },
   notifications: true,
+  temp_unit: 'C',
 };
 
 export async function getBriefingConfig(): Promise<BriefingConfig> {