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/lib/stores/appSettings.ts

@@ -0,0 +1,28 @@
+import { writable } from 'svelte/store';
+import { browser } from '$app/environment';
+
+export interface AppSettings {
+	fileLoadLimit: number;
+	tagRuleApplyToExisting: boolean;
+}
+
+const DEFAULTS: AppSettings = {
+	fileLoadLimit: 100,
+	tagRuleApplyToExisting: false,
+};
+
+function load(): AppSettings {
+	if (!browser) return { ...DEFAULTS };
+	try {
+		const stored = JSON.parse(localStorage.getItem('app-settings') ?? 'null');
+		return stored ? { ...DEFAULTS, ...stored } : { ...DEFAULTS };
+	} catch {
+		return { ...DEFAULTS };
+	}
+}
+
+export const appSettings = writable<AppSettings>(load());
+
+appSettings.subscribe((v) => {
+	if (browser) localStorage.setItem('app-settings', JSON.stringify(v));
+});