feat(mobile): compact play layout, touch stick, and action pad (#21)

[Aduersarius]

Summary

Ships mobile playability for #21 so /play is usable on a phone without changing desktop.

• Compact play layout: coarse-pointer / narrow-touch detection scales the fixed 21×21 canvas to the viewport (same CSS stretch model as fullscreen). Side inventory becomes an overlay sheet (Panel) instead of requiring ~1050px width.
• Virtual stick → real movement path: useTouchStickMovement updates the same movementPressCountsRef / movementKeyPriorityRef the keyboard uses, via a dedicated __touch_stick__ source code. No synthetic KeyboardEvents (blocked by isTrusted). Engine throttle / inmo / para unchanged.
• Touch action cluster: attack, spell (selected / opens panel), use item, pickup, seguro, chat, panel — wired to existing packets / requests.
• Long-press = right-click on the map (touch/pen only); mouse path unchanged.
• Landscape gate overlays in portrait without remounting the session tree (rotation must not drop the socket).
• Stuck-input clear on visibilitychange / pagehide / blur.
• PWA: viewport-fit: cover, non-scalable viewport, manifest orientation: "landscape".
• Pure helpers + node --experimental-strip-types --test frontend/lib/mobile/playability.test.ts.

Differentiation vs open PRs

PR	Gap vs this
#42 (waterWang)	Joystick-only MVP; CSS max-width hack; clears all movement counts on turn; no action pad / long-press / compact HUD overlay; LandscapePrompt mounted awkwardly.
#61 (angelTomo9)	Kitchen-sink (protocol, map moderation, converter, …); MobileTouchControls UI is not wired into MapRenderer movement/combat; conflicting with main. This PR is #21-only and hooks the real input/combat paths.

Test plan

• node --experimental-strip-types --test frontend/lib/mobile/playability.test.ts
• Phone landscape: canvas fits; stick walks at keyboard step rate; attack/hechizo/usar/chat/panel work
• Portrait: rotate overlay; rotate back without reconnect
• Long-press tile ≈ right-click; tap still left-click / walk
• Background the tab / lock screen → movement not stuck on return
• Desktop fine-pointer: unchanged layout, no stick/action pad

Tracks #21.

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