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/admin/BusinessSettings.svelte

@@ -118,10 +118,14 @@
 	}
 
 	function validateVatNumber(value: unknown): string {
-		const v = value === null || value === undefined ? '' : String(value);
-		const trimmed = v.trim();
+		const trimmed = String(value ?? '').trim();
 		if (!trimmed) return '';
-		if (trimmed.length > 20) return 'Must be 20 characters or fewer';
+		// UK VAT numbers: GB + 9 digits (standard) or GB + 12 digits (branch)
+		if (trimmed.length !== 11 && trimmed.length !== 14) return 'Must be GB followed by 9 or 12 digits';
+		if (!trimmed.startsWith('GB')) return 'Must start with GB';
+		const digits = trimmed.slice(2);
+		if (!/^\d+$/.test(digits)) return 'Must contain only digits after GB';
+		if (digits.length !== 9 && digits.length !== 12) return 'Must be GB followed by 9 or 12 digits';
 		return '';
 	}
 
@@ -207,7 +211,7 @@
 			];
 
 			for (const field of fields) {
-				const newVal = normalisedForm[field] as unknown;
+				const newVal = normalisedForm[field];
 				const oldVal = settings[field];
 				if (JSON.stringify(newVal) !== JSON.stringify(oldVal)) {
 					patch[field] = newVal;