feat: Ren'Py screens (screen/call screen/show screen) with actions and styles

Converter
- screens.py parses the screen language: containers (fixed, frame, vbox, hbox,
  grid, vpgrid, viewport, button, has), widgets (text, textbutton, imagebutton,
  add, bar ...), if/for/use/transclude, default, python, key, timer, on, inline ATL,
  conditional properties, and decompiled forms (unparenthesised operators,
  glued property values, comma separated actions)
- style statements, call/show/hide screen operators and Python-level
  renpy.show_screen are translated; only screens the game shows are audited,
  unused stock screens no longer block an import; undefined screens do
- ATL values may be computed at run time and transforms take parameters
  (at blink(0.2, 0.3)); blur property

Player
- screens.ts builds the DOM from the IR and rebuilds it on state changes;
  buttons with idle/hover/selected/insensitive states, tooltips, sounds, modal
  screens, timers and keys that survive re-rendering, saved with the save file
- Actions: Jump, Call, Return, SetVariable, Toggle*, SetScreenVariable, Show, Hide,
  ToggleScreen, Function, Play, Notify, If, Language, ShowMenu, OpenURL ...
- Crop/LiveCrop/Solid/Frame/Transform/DynamicDisplayable values, crop property,
  globals(), renpy.variant/change_language/seen_image, transition stubs

Tests: screen parser and runtime (linkedom DOM), screens fixture compiled from
tests/fixtures/screens.rpy. Docs: docs/SCREENS.md.

Twisted-Thoughts now passes the strict compatibility check.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

frontend/src/player/player.css

@@ -343,3 +343,20 @@
 .pg-dialog-row { display: flex; justify-content: flex-end; gap: 10px; }
 .pg-dialog button { font-size: 15px; padding: 9px 18px; }
 .pg-dialog .pg-primary { background: #ff9f43; border-color: #ff9f43; color: #1a1205; font-weight: 650; }
+
+/* --- game screens (Ren'Py screen language) ------------------------------------------ */
+.pg-usr { position: absolute; inset: 0; pointer-events: none; }
+.pg-sinst { position: absolute; inset: 0; pointer-events: none; }
+.pg-sinst.pg-smodal { pointer-events: auto; }
+.pg-sw { pointer-events: none; min-width: 0; min-height: 0; color: var(--pg-text, #fff); font-size: var(--pg-font, 24px); }
+.pg-sframe, .pg-swindow, .pg-sviewport, .pg-sbar, .pg-sbtn { pointer-events: auto; }
+.pg-sbtn { cursor: pointer; user-select: none; }
+.pg-sbtn.pg-sdisabled { cursor: default; }
+.pg-sbtn:focus-visible { outline: 2px solid var(--pg-accent, #4af); outline-offset: 2px; }
+.pg-simgbtn img, .pg-simgbtn .pg-simg { display: block; pointer-events: none; }
+.pg-stext { white-space: pre-wrap; }
+.pg-sbar { background: rgba(255, 255, 255, 0.18); min-width: 40px; min-height: 12px; }
+.pg-sbar-fill { background: var(--pg-accent, #4af); width: 100%; height: 100%; }
+.pg-sbar:not(.pg-svbar) .pg-sbar-fill { height: 100%; }
+.pg-sadd { pointer-events: none; }
+.pg-sviewport { scrollbar-width: thin; }