fix(android): default to classic theme to fix AMOLED card-back invisibility

Dark theme back.svg uses #151515 (near-black) as the card back background,
which AMOLED screens render as fully-off pixels, leaving only the tiny
#a54242 red badge visible — user sees solid red squares instead of card backs.

Fix: change fresh-install default theme from "dark" to "classic" (white
background with navy diamond pattern, clearly visible on all display types).
Also remove the stale "classic" -> "dark" sanitize migration, correct wrong
asset paths in load_card_images (classic/ subdirectory was missing), and
update tests that hardcoded the old TABLEAU_FAN_FRAC=0.25 constant.

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

solitaire_engine/src/card_plugin.rs

@@ -484,13 +484,13 @@ fn load_card_images(asset_server: Option<Res<AssetServer>>, mut commands: Comman
     let faces: [[Handle<Image>; 13]; 4] = std::array::from_fn(|suit| {
         std::array::from_fn(|rank| {
             asset_server.load(format!(
-                "cards/faces/{}{}.png",
+                "cards/faces/classic/{}{}.png",
                 RANK_STRS[rank], SUIT_CHARS[suit]
             ))
         })
     });
     let backs = std::array::from_fn(|i| {
-        asset_server.load(format!("cards/backs/back_{i}.png"))
+        asset_server.load(format!("cards/backs/classic/back_{i}.png"))
     });
     commands.insert_resource(CardImageSet {
         faces,