feat(frontend): visual QA pass — unified page titles, CTAs, native controls, 404 fixes

- add PageTitle and apply it on /download, /themes, /configs, /showcase, /link, 404
- reset range-slider thumbs in the theme editor; fold color picker into the swatch
- hide horizontal scrollbars in the ClickGui mock; clip html to kill 100vw pan
- pin the footer under short pages via flex AppShell; split 404 art from caption
- one landing CTA geometry (lv-cta-lg / quiet); CLOUD eyebrow; auth caption scrim
- showcase error hides sort/pager; query retry limited to 1

frontend/src/index.css

@@ -40,6 +40,9 @@
     --topbar-h: 4.25rem;
     scroll-padding-top: calc(var(--topbar-h) + 1rem);
     color-scheme: dark;
+    /* 100vw full-bleed sections exceed the scrollbar-less client width by a few px;
+       clip on html too so the document can never pan horizontally. */
+    overflow-x: clip;
     transition: --color-ice 400ms ease, --page-accent-glow 400ms ease;
   }
   body {