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/themes/editor/fields/GradientField.tsx

@@ -36,7 +36,7 @@ export function GradientField({ title, value, onChange }: Props) {
               max={359}
               value={Math.round(value.angle)}
               onChange={(e) => onChange({ angle: Number(e.target.value) })}
-              className="h-1.5 flex-1 cursor-pointer appearance-none rounded-full bg-white/20"
+              className="lv-alpha-slider h-1.5 flex-1 cursor-pointer appearance-none rounded-full bg-white/20"
             />
             <span className="w-10 shrink-0 text-right font-code text-snow">{Math.round(value.angle)}°</span>
           </label>