Fix dogfood QA: 404, content hygiene, grid UX, a11y, CTAs

- Add branded NotFound catch-all (no more blank unknown routes)
- Portfolio cards: shimmer skeleton, eager first-row thumbs, real alts
- Hide empty categories; rename CAD view slug to avoid collision
- Resume summary preserves paragraphs/bullets
- Home/About/footer hire contact mailto CTA
- Sticky header scroll-padding; chip aria-labels
- Security headers; Flickr import strip View On Black + junk tags
- Content cleanup script for grammar, residue, tags, featured

client/src/styles/global.scss

@@ -31,6 +31,8 @@
 
 html {
   scroll-behavior: smooth;
+  /* Keep anchored content clear of sticky site header */
+  scroll-padding-top: calc(var(--header-h) + 0.75rem);
 }
 
 body {