refactor(frontend): timezone-safe date handling with London-aware utilities

Introduce getLondonTodayCalendarDate(), parseWallClockDate(), and formatLocalDateTime() for reliable Europe/London timezone handling. Replace ad-hoc SvelteDate/new Date() usage with these utilities across all components and stores.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

frontend/src/lib/components/ui/map/MapArc.svelte

@@ -231,8 +231,7 @@
 		if (!map || !isLoaded) return;
 		if (map.getLayer(layerId)) {
 			for (const [key, value] of Object.entries(mergedPaint)) {
-				// eslint-disable-next-line @typescript-eslint/no-explicit-any
-				map.setPaintProperty(layerId, key as any, value);
+				map.setPaintProperty(layerId, key as keyof MapArcLinePaint, value);
 			}
 		}
 	});