fix(ext/sw): inactivity timer resets on all non-passive messages (Plan C Phase 5)

DEV-C P2: an active autofiller never opens the popup, so under the old
rule it got force-locked despite continuous use. Inverts the rule:
reset on all messages except a documented exclusion set (only
get_autofill_candidates today).

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

extension/src/service-worker/session-timer.ts

@@ -48,3 +48,17 @@ export function stopTimer(): void {
     timerId = null;
   }
 }
+
+/**
+ * Content-callable message types that should NOT reset the inactivity timer.
+ *
+ * Rationale: a content script reading available autofill candidates is a
+ * passive query — it shouldn't keep the vault alive indefinitely while the
+ * user isn't actually interacting with it.
+ *
+ * Today this is the only known passive read; if a future content message
+ * is also passive, add it here with a one-line justification.
+ */
+export const READ_ONLY_CONTENT_CALLABLE: ReadonlySet<string> = new Set([
+  'get_autofill_candidates',
+]);