feat(web): add classic/dark card theme picker

- Reorganise card PNGs into assets/cards/faces/{classic,dark}/ and
  assets/cards/backs/{classic,dark}/
- Rasterise dark SVG theme alongside existing classic set
- Add "Dark / Classic" toggle button in the game HUD; persists to
  localStorage as fs_theme (defaults to classic)
- Preload both themes on page load so switching is instant

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

scripts/gen_classic_pngs.sh

@@ -1,33 +1,33 @@
 #!/usr/bin/env bash
-# Rasterize the classic SVG theme into the web game's card PNG assets.
+# Rasterize card SVG themes into the web game's PNG asset subdirectories.
+# Usage: bash scripts/gen_classic_pngs.sh
+# Outputs: assets/cards/faces/{classic,dark}/ and assets/cards/backs/{classic,dark}/
 set -euo pipefail
 
-CLASSIC="solitaire_engine/assets/themes/classic"
-OUT="assets/cards/faces"
-BACKS_OUT="assets/cards/backs"
-
 declare -A SUIT=([clubs]=C [diamonds]=D [hearts]=H [spades]=S)
 declare -A RANK=([ace]=A [2]=2 [3]=3 [4]=4 [5]=5 [6]=6 [7]=7 [8]=8 [9]=9 [10]=10 [jack]=J [queen]=Q [king]=K)
 
-mkdir -p "$OUT" "$BACKS_OUT"
+rasterize_theme() {
+    local theme="$1"
+    local src="solitaire_engine/assets/themes/$theme"
+    local faces_out="assets/cards/faces/$theme"
+    local backs_out="assets/cards/backs/$theme"
+    mkdir -p "$faces_out" "$backs_out"
 
-for svg in "$CLASSIC"/*_*.svg; do
-    base=$(basename "$svg" .svg)          # e.g. clubs_ace
-    suit_name="${base%%_*}"               # clubs
-    rank_name="${base#*_}"                # ace
-    suit_code="${SUIT[$suit_name]:-}"
-    rank_code="${RANK[$rank_name]:-}"
-    if [ -z "$suit_code" ] || [ -z "$rank_code" ]; then
-        echo "skip: $base"
-        continue
-    fi
-    out="$OUT/${rank_code}${suit_code}.png"
-    rsvg-convert -w 256 -h 384 "$svg" -o "$out"
-    echo "  $base -> $out"
-done
+    for svg in "$src"/*_*.svg; do
+        local base suit_name rank_name suit_code rank_code
+        base=$(basename "$svg" .svg)
+        suit_name="${base%%_*}"
+        rank_name="${base#*_}"
+        suit_code="${SUIT[$suit_name]:-}"
+        rank_code="${RANK[$rank_name]:-}"
+        [ -z "$suit_code" ] || [ -z "$rank_code" ] && continue
+        rsvg-convert -w 256 -h 384 "$svg" -o "$faces_out/${rank_code}${suit_code}.png"
+    done
 
-# Back
-rsvg-convert -w 256 -h 384 "$CLASSIC/back.svg" -o "$BACKS_OUT/back_0.png"
-echo "  back -> $BACKS_OUT/back_0.png"
+    rsvg-convert -w 256 -h 384 "$src/back.svg" -o "$backs_out/back_0.png"
+    echo "done: $theme ($(ls "$faces_out" | wc -l) faces)"
+}
 
-echo "Done."
+rasterize_theme classic
+rasterize_theme dark