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;

extension/src/popup/components/settings.ts

@@ -53,13 +53,29 @@ export async function renderSettings(container: HTMLElement): Promise<void> {
   await renderSection(activeSection);
 }
 
-export function teardownSettings(): void {
+/**
+ * Common cleanup invoked by both the device-settings teardown
+ * (settings.ts) and the vault-settings teardown (settings-vault.ts).
+ * Centralized to avoid the "regression class with known prior leaks"
+ * DEV-C P2 flagged.
+ *
+ * Closes the generator popover and detaches the supplied keydown
+ * handler from the document if present. Returns the new handler value
+ * (always null), so the caller can do `handler = teardownSettingsCommon(handler)`.
+ */
+export function teardownSettingsCommon(
+  keyHandler: ((e: KeyboardEvent) => void) | null,
+): null {
   closeGeneratorPanel();
-  teardownSecuritySection();
-  if (activeKeyHandler) {
-    document.removeEventListener('keydown', activeKeyHandler);
-    activeKeyHandler = null;
+  if (keyHandler) {
+    document.removeEventListener('keydown', keyHandler);
   }
+  return null;
+}
+
+export function teardownSettings(): void {
+  activeKeyHandler = teardownSettingsCommon(activeKeyHandler);
+  teardownSecuritySection();
   pendingVaultSettings = null;
   sessionHandle = null;
 }