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/today/CurrentAppointment.svelte

@@ -148,7 +148,7 @@
 				const today = new SvelteDate();
 				const closing = new SvelteDate(
 					today.getFullYear(),
-					today.getMonth() + 1,
+					today.getMonth(),
 					today.getDate(),
 					ch,
 					cm,