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/components/blog/MermaidRenderer.tsx

@@ -8,12 +8,10 @@ function sanitizeMermaidSvg(svgString: string): string {
   const parser = new DOMParser();
   const doc = parser.parseFromString(svgString, "text/html");
 
-  // Remove dangerous elements
   doc.querySelectorAll("script, iframe, object, embed, form, link, meta, base").forEach((el) => {
     el.remove();
   });
 
-  // Remove event handlers and dangerous attributes from all elements
   doc.querySelectorAll("[on*], [href*='javascript:'], [style*='expression(']").forEach((el) => {
     const attrs = Array.from(el.attributes);
     attrs.forEach((attr) => {