feat(mvp): phase 8 - integration, testing & deployment

Fix all build/type/lint errors (zod 3.25 compat wrapper, Svelte 5 fixes),
write 115 unit tests across 10 test files, expand seed script with demo
data, update Docker config with migration on startup.

src/lib/stores/theme.svelte.ts

@@ -35,14 +35,14 @@ class ThemeStore {
 	primarySaturation = $state(70);
 	backgroundType = $state<BackgroundType>('mesh');
 
+	#systemPreference: 'dark' | 'light' = 'dark';
+
 	resolvedMode = $derived<'dark' | 'light'>(
 		this.mode === 'system' ? this.#systemPreference : this.mode
 	);
 
 	isDark = $derived(this.resolvedMode === 'dark');
 
-	#systemPreference: 'dark' | 'light' = 'dark';
-
 	constructor() {
 		if (typeof window !== 'undefined') {
 			this.mode = getStoredValue<ThemeMode>(THEME_STORAGE_KEY, 'system');