refactor(ext/popup): extract teardownSettingsCommon (Plan C Phase 5)

DEV-C P2: settings.ts:56-65 and settings-vault.ts:15-22 had near-
identical cleanup paths. Single source for closeGeneratorPanel +
activeKeyHandler removal. Helper takes the handler as a parameter and
returns null so each caller still owns its own module-scoped handler
state.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

extension/src/popup/components/settings-vault.ts

@@ -9,6 +9,7 @@ import type {
 import type { SessionTimeoutConfig } from '../../shared/messages';
 import { relativeTime } from '../../shared/relative-time';
 import { openGeneratorPanel, closeGeneratorPanel, isGeneratorPanelOpen } from './generator-panel';
+import { teardownSettingsCommon } from './settings';
 import { GLYPH_NEXT } from '../../shared/glyphs';
 
 let pendingSettings: VaultSettings | null = null;
@@ -17,11 +18,7 @@ let pendingSession: SessionTimeoutConfig | null = null;
 let baseSession: SessionTimeoutConfig | null = null;
 
 export function teardown(): void {
-  closeGeneratorPanel();
-  if (activeKeyHandler) {
-    document.removeEventListener('keydown', activeKeyHandler);
-    activeKeyHandler = null;
-  }
+  activeKeyHandler = teardownSettingsCommon(activeKeyHandler);
   pendingSettings = null;
   pendingSession = null;
   baseSession = null;