chore: remediate pygienium audit findings

Dead code: 77 verified-unused exports, files (BackArrow, MenuBars,
cookies.ts, db/create.ts, schemas/comment.ts, security-headers.ts) and
12 unused dependencies removed
Comments: ~370 RESTATE comments stripped across 53 files; 2 verbose
blocks tightened; dead commented-out config removed
Complexity: bulkUpsert extracted into 11 per-entity helpers (CCN 156->~10);
login formHandler split into 3 submitters (CCN 63->~5); account page render
split into 8 section components (CCN 40); updatePost SQL builder rebuilt;
assert*Owned consolidated behind generic assertOwnedBy
Defensive guards: 4 redundant rethrow/nullish guards removed

src/lib/useCountdown.ts

@@ -45,15 +45,12 @@ export function useCountdown(options: UseCountdownOptions = {}) {
   };
 
   const startCountdown = (expiresAt: string | Date) => {
-    // Clear any existing interval
     if (intervalId !== null) {
       clearInterval(intervalId);
     }
 
-    // Calculate immediately
     calculateRemaining(expiresAt);
 
-    // Then update every second
     intervalId = setInterval(() => calculateRemaining(expiresAt), 1000);
   };
 
@@ -64,7 +61,6 @@ export function useCountdown(options: UseCountdownOptions = {}) {
     }
   };
 
-  // Cleanup on unmount
   onCleanup(() => {
     stopCountdown();
   });