[Fonts] Retire max font size setting into theme CardText min/max range

The per-card text size lives in the theme now, so a theme can pick the size
it was designed for instead of every user overriding it globally.

- CardText stores family + max point size; CardTextMinSize stores the
  overlay shrink-to-fit floor (omitted from fonts.toml when 1).
- ThemeManager::cardTextMaxSize()/cardTextMinSize() resolve the range with
  the retired defaults (12 max, 1 min), and cardTextMaxSize() falls back to
  the CardText role size that #7541 made live.
- The overlay widget starts at the explicit size when one is set (deck
  preview), else the theme max, and shrinks down to the theme floor.
- The appearance page's "Maximum font size" spin is gone; the range is
  edited in the theme's font grid, whose min/max spins now keep the saved
  range consistent with each other.
- Migration: appearance/maxFontSize is seeded into every user theme folder
  and into the active theme's folder (created when the theme is a shipped
  one that only exists in the system directory). The retired
  game/maxfontsize -> appearance/maxFontSize mapping stays, so users still
  on global.ini keep their value through the chain.

libcockatrice_settings/libcockatrice/settings/settings_migration.h

@@ -18,6 +18,15 @@ public:
      */
     static bool migrateGameFiltersToCamelCase(const QString &settingsPath);
 
+    /**
+     * Moves a non-default appearance/maxFontSize value into each user-writable theme
+     * directory lacking an explicit CardText max (creating/merging fonts.toml, never
+     * overwriting an explicit max), then deletes the legacy key. Default (12) or absent
+     * values only delete the key and create no files, since the ThemeManager fallback
+     * already covers them. Idempotent, safe to run on every startup.
+     */
+    static bool migrateAppearanceMaxFontSizeToTheme(const QString &settingsPath);
+
     /**
      * Returns true when at least one of the two migrations would still do work, i.e. when a
      * backup of the settings directory is worth taking before they run.