fix(android): 48dp min hit targets on action buttons

Action buttons sized to text + 8 px padding made "Undo" end up
~46 x 33 px — fine for a mouse but at the threshold of a finger.
Adds `min_width: 48 px` and `min_height: 48 px` to the button
Node so every button meets Material's 48 dp thumb-target guideline.

Applied universally; the floor is a no-op for buttons whose
content already exceeds 48 px on either axis (Menu, Modes,
New Game, Pause, Help all clear 48 px wide; height was the
binding constraint at ~33 px).

Closes P1 #2 of docs/android/PLAYABILITY_TODO.md. Engine tests
pass; clippy clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/android/PLAYABILITY_TODO.md

@@ -76,9 +76,14 @@ rewrites required.
   profile/stats/leaderboard/settings, help screen) survive — they
   live behind navigation and a touch user reaches them less often.
   Track as a P3 sweep when more screens are audited on hardware.
-- [ ] **Thumb-sized hit targets.** HUD buttons sized for mouse;
-  Material guideline minimum is 44–48 dp. Increase button paddings
-  on touch builds.
+- [x] **Thumb-sized hit targets.** *Closed 2026-05-10.* Action
+  button Node carries `min_width: Val::Px(48.0), min_height:
+  Val::Px(48.0)` — meets Material's 48 dp baseline on touch and is
+  a no-op for buttons whose content already exceeds 48 px in
+  either axis. Applied universally rather than cfg-gated since
+  Material's guideline applies to all input modes. Cards, pile
+  markers, modal close buttons not yet audited — track as P3 if
+  they fall below threshold on hardware.
 - [ ] **Portrait-first card spacing.** Stretch tableau piles vertically
   to fill height; reduce inter-pile gaps so 7 columns fit in 360 dp.
 - [ ] **Double-tap auto-move visible feedback.** `handle_double_tap`