feat(frontend): default the apply-to-existing tag-rule toggle on

New tag rules should retroactively apply to existing files by default, so flip
the tagRuleApplyToExisting default to true. (Settings already saved in
localStorage keep their stored value.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/stores/appSettings.ts

@@ -8,7 +8,7 @@ export interface AppSettings {
 
 const DEFAULTS: AppSettings = {
 	fileLoadLimit: 100,
-	tagRuleApplyToExisting: false
+	tagRuleApplyToExisting: true
 };
 
 function load(): AppSettings {