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/features/landing/i18n/en.ts

@@ -27,6 +27,7 @@ export const landingEn = {
     palette: 'Theme palette',
   },
   cloud: {
+    eyebrow: 'CLOUD',
     headlinePart1: 'Your LoVisual settings',
     headlinePart2: 'on any computer',
     subtitle: 'Four cloud slots, a code you can share with friends, and a showcase of ready-made configs.',