fix(ui): meet 44px touch-target floor on shared mobile primitives

The desktop/mobile consistency review found primary mobile controls
rendering below the ~44px comfortable touch floor. Address the genuinely
shared primitives in one pass:

- Add a --tap-min: 44px token (documented anchor for the floor).
- tokens.css: text inputs, selects, and submit buttons grow to --tap-min
  under the 640px breakpoint (fixes the 36px auth/login form controls).
  Scoped to form controls + type=submit so dense icon buttons aren't
  inflated.
- SegmentedControl: .seg options reach the floor on mobile (it doubles as
  the catalog sort toggle and the Library tab switcher).
- Chip: expand .chip-remove's tappable area to --tap-min via a centered
  pseudo-element so the 16px glyph stays compact and tag rows don't grow.

Pure-CSS responsive change — jsdom can't evaluate @media or layout, so the
test pins the stylesheet contract (each shared primitive bumps to --tap-min
inside the mobile breakpoint), guarding against the bump being dropped.

The per-route .icon-btn (copy-pasted across seven files) is left for a
separate refactor.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/styles/tokens.css

@@ -78,6 +78,12 @@
     --bp-md: 768px;
     --bp-lg: 1024px;
 
+    /* Comfortable touch-target floor for primary controls on phones.
+       Controls compact to their desktop height by default and grow to
+       this minimum under the 640px breakpoint. See touch-targets.test.ts
+       for the shared-primitive contract. */
+    --tap-min: 44px;
+
     /* Safe-area helpers for notched / home-indicator devices. Resolve
        to 0 on devices without insets, so they're safe everywhere as
        long as <meta name="viewport" content="…, viewport-fit=cover">
@@ -293,6 +299,23 @@ img {
     body {
         overflow-x: hidden;
     }
+
+    /* Touch-target floor on phones. Form controls and submit buttons keep
+       their compact 36px desktop height above the breakpoint but grow to a
+       comfortable tap size here. `min-height` (not `height`) so the
+       already-tall textarea is unaffected. Scoped to text fields, selects,
+       and `type=submit` deliberately — bare `button` would also catch dense
+       icon buttons (chip remove, segmented options, reader chrome) that meet
+       the floor through their own hit-area rules instead. */
+    input[type='text'],
+    input[type='search'],
+    input[type='password'],
+    input[type='number'],
+    input[type='email'],
+    select,
+    button[type='submit'] {
+        min-height: var(--tap-min);
+    }
 }
 
 @media (prefers-reduced-motion: reduce) {