Estil targeta tipus web a les files + tema seleccionable (system/clar/fosc)

UI:
- Files amb estil de targeta: icona arrodonida, títol gran, subtítol atenuat i
  'pills' amb estat, versió, data de llançament, jugadors, autor i topics. Els
  pills envolten amb un FlowLayout nou quan no caben.
- Submenú Opcions > Tema amb Sistema/Clar/Fosc; persisteix a settings.json
  (theme) i s'aplica a l'instant. El watcher del SO només actua en mode Sistema.

Dades:
- GameMeta guarda topics i created_at, llegits de la resposta de Gitea que ja
  demanàvem (gratis, auto-sincronitzats).
- games.toml: camps opcionals players i author per joc (la resta surt de Gitea).

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

jlauncher/ui/main_window.py

@@ -5,8 +5,9 @@ from __future__ import annotations
 from pathlib import Path
 
 from PySide6.QtCore import QThreadPool, Qt, QTimer
-from PySide6.QtGui import QAction
+from PySide6.QtGui import QAction, QActionGroup
 from PySide6.QtWidgets import (
+    QApplication,
     QInputDialog,
     QLineEdit,
     QMainWindow,
@@ -23,6 +24,7 @@ from .. import gitops
 from ..config import Config, Game
 from ..settings import load_settings, save_settings
 from ..workers import CheckUpdatesWorker, DownloadWorker, RunWorker
+from . import theme
 from .game_row import GameRow
 
 APP_NAME = "Jail Launcher"
@@ -46,6 +48,13 @@ class MainWindow(QMainWindow):
         self.setWindowTitle(WINDOW_TITLE)
         self.resize(720, 640)
 
+        # Aplica el tema guardado (system/light/dark) i vigila els canvis del SO
+        # només quan estem en mode 'system'.
+        app = QApplication.instance()
+        if app is not None:
+            theme.apply_theme(app, self.settings.theme)
+            theme.watch_system_theme(app, lambda: self.settings.theme == theme.THEME_SYSTEM)
+
         self._build_menu()
 
         splitter = QSplitter(Qt.Vertical)
@@ -130,6 +139,9 @@ class MainWindow(QMainWindow):
         self.action_check_on_start.toggled.connect(self._on_toggle_check_on_start)
         menu.addAction(self.action_check_on_start)
 
+        menu.addSeparator()
+        self._build_theme_menu(menu)
+
         menu.addSeparator()
         self.action_delete = QAction("Esborra un joc", self, checkable=True)
         self.action_delete.toggled.connect(self._set_delete_mode)
@@ -140,6 +152,30 @@ class MainWindow(QMainWindow):
         self.action_token.triggered.connect(self._configure_token)
         menu.addAction(self.action_token)
 
+    def _build_theme_menu(self, parent_menu) -> None:
+        """Submenú Tema amb tres opcions exclusives: Sistema / Clar / Fosc."""
+        submenu = parent_menu.addMenu("Tema")
+        group = QActionGroup(self)
+        group.setExclusive(True)
+        options = [
+            ("Sistema", theme.THEME_SYSTEM),
+            ("Clar", theme.THEME_LIGHT),
+            ("Fosc", theme.THEME_DARK),
+        ]
+        for label, mode in options:
+            action = QAction(label, self, checkable=True)
+            action.setChecked(self.settings.theme == mode)
+            action.triggered.connect(lambda _checked, m=mode: self._on_theme_selected(m))
+            group.addAction(action)
+            submenu.addAction(action)
+
+    def _on_theme_selected(self, mode: str) -> None:
+        self.settings.theme = mode
+        save_settings(self.settings)
+        app = QApplication.instance()
+        if app is not None:
+            theme.apply_theme(app, mode)
+
     def _configure_token(self) -> None:
         token, ok = QInputDialog.getText(
             self,