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/date-utils.ts

@@ -81,7 +81,6 @@ export function formatRelativeTime(
       return `${diffDay}d ago`;
     }
   } else {
-    // style === "long"
     if (includeSeconds && diffSec < 60) {
       return `${diffSec} second${diffSec === 1 ? "" : "s"} ago`;
     }