feat(design): the field family, the dropdown trigger, native controls, the no-raw-input lint rule (E28 S3c)

- ui/Input, Textarea, InnerInput + fieldClass (A4): the >= 3:1 edge, a 2px accent
  focus edge, the danger edge for invalid, md/sm/xs/heading rungs, 1rem and 44 on touch
- ui/SelectTrigger (A5): SortControl, FilterSelect, the view switcher and the date
  filter on one trigger, a pill in a toolbar, r8 in a form
- ui/NativeControls (A6): Checkbox, CheckRow, Slider, ColorInput
- every raw input swept (~100 sites); inputCls retired; local/input-primitive
- useReturnFocus: an in-place editor hands the focus back to its trigger on
  Enter/Escape (the playlist and Plex title renames, the priority number);
  the Plex playlist rename gains Escape and an accessible name
- back to top: a round 48 FAB (owner decision); DESIGN.md A2 and the log

frontend/src/components/ui/BackToTop.tsx

@@ -50,15 +50,17 @@ export default function BackToTop({ dep, threshold = 600 }: { dep?: unknown; thr
   // No aria-label/title attributes: ad-block "annoyance" cosmetic filters (e.g. Brave Shields)
   // commonly hide floating corner buttons via attribute selectors like [aria-label="Back to top"].
   // The accessible name comes from a visually-hidden child instead, which those selectors can't match.
+  // Shape (docs/DESIGN.md A2): round because it floats over the content, 3rem (48) on mouse and touch —
+  // a lone corner button at a toolbar rung gets lost.
   return createPortal(
     <Pressable
       align="center"
       onClick={toTop}
-      className={`fixed bottom-5 right-5 z-chrome inline-flex items-center justify-center w-16 h-16 rounded-full glass-card glass-hover text-fg hover:text-accent transition-all duration-200 ${
+      className={`fixed bottom-5 right-5 z-chrome inline-flex items-center justify-center size-12 rounded-full glass-card glass-hover text-fg hover:text-accent transition-all duration-200 ${
         show ? "opacity-100 translate-y-0" : "opacity-0 translate-y-3 pointer-events-none"
       }`}
     >
-      <ArrowUp className="w-7 h-7" aria-hidden="true" />
+      <ArrowUp className="size-5" aria-hidden="true" />
       <span className="sr-only">{t("common.backToTop")}</span>
     </Pressable>,
     document.body,