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/context/SiteContext.tsx

@@ -58,7 +58,7 @@ declare global {
 }
 
 /** Resolve the client-side active site, preferring the SSR-injected id. */
-export function resolveClientSite(): Site {
+function resolveClientSite(): Site {
   if (typeof window === "undefined") return MAIN_SITE;
   const injected = window.__SITE__;
   if (injected && SITE_CONFIG[injected]) return SITE_CONFIG[injected];