feat(frontend): add configurable app settings (file load limit, tag rule apply_to_existing)

- Add appSettings store (localStorage-backed) with two settings:
  fileLoadLimit (default 100) and tagRuleApplyToExisting (default false)
- Settings page: new Behaviour section with numeric input for files per
  page (10–500) and an on/off toggle for retroactive tag rule application
- files/+page.svelte: derive LIMIT from appSettings.fileLoadLimit so
  changes take effect immediately without reload
- TagRuleEditor: pass apply_to_existing from appSettings when activating
  a rule via PATCH (only sent on activation, not deactivation)

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

frontend/src/routes/files/+page.svelte

@@ -15,6 +15,7 @@
 	import BulkTagEditor from '$lib/components/file/BulkTagEditor.svelte';
 	import { parseDslFilter } from '$lib/utils/dsl';
 	import type { File, FileCursorPage, Pool, PoolOffsetPage } from '$lib/api/types';
+	import { appSettings } from '$lib/stores/appSettings';
 
 	let uploader = $state<{ open: () => void } | undefined>();
 	let confirmDeleteFiles = $state(false);
@@ -65,7 +66,7 @@
 		files = [file, ...files];
 	}
 
-	const LIMIT = 50;
+	let LIMIT = $derived($appSettings.fileLoadLimit);
 
 	const FILE_SORT_OPTIONS = [
 		{ value: 'created', label: 'Created' },