feat(web): add solitaire_web Bevy WASM build targeting play.html canvas

Adds a new `solitaire_web` crate that compiles the full `solitaire_engine`
to `wasm32-unknown-unknown` and renders to a `<canvas id="bevy-canvas">`
element in `play.html` — the same ECS code path as desktop and Android.

Changes to enable the WASM target:
- .cargo/config.toml: add wasm32-unknown-unknown rustflags for getrandom
- Workspace Cargo.toml: add solitaire_web member
- solitaire_data/Cargo.toml: gate tokio/reqwest/dirs/keyring to non-wasm
- solitaire_data/src: add wasm32 branch to data_dir() (returns None);
  cfg-gate sync_client network types, auth_tokens, matomo_client
- solitaire_engine/Cargo.toml: gate tokio/reqwest/kira/arboard/dirs/zip
  to non-wasm (mio/cpal/arboard don't compile for wasm32-unknown-unknown)
- solitaire_engine/src/lib.rs: cfg-gate module declarations and re-exports
  for analytics, audio, sync, sync_setup, avatar, leaderboard plugins
- solitaire_engine/src/core_game_plugin.rs: cfg-gate plugin registrations
  that require TokioRuntime (audio, sync, analytics, leaderboard, avatar)
- solitaire_engine/src/resources.rs: cfg-gate TokioRuntimeResource
- solitaire_engine/src/game_plugin.rs: cfg-gate std::fs::remove_file (x10)
- solitaire_engine/src/theme/mod.rs: cfg-gate importer module (uses dirs+zip)
- solitaire_engine/src/settings_plugin.rs: cfg-gate theme ZIP import UI
- solitaire_engine/src/assets/sources.rs: cfg-gate FileAssetReader/user_theme_dir
- solitaire_engine/src/auto_complete_plugin.rs: cfg-gate audio system
- solitaire_engine/src/daily_challenge_plugin.rs: cfg-gate server fetch
- solitaire_engine/src/hud_plugin.rs: cfg-gate AvatarResource import
- solitaire_engine/src/profile_plugin.rs: cfg-gate AvatarResource import
- solitaire_server/web/play.html: minimal HTML canvas shell
- solitaire_web/: new crate (Cargo.toml + src/lib.rs)
- build_wasm.sh: add Bevy WASM build step (cargo + wasm-bindgen + wasm-opt)

All tests pass; clippy --workspace -- -D warnings clean; native build
(solitaire_engine, solitaire_app) unaffected.

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

solitaire_engine/src/assets/sources.rs

@@ -50,9 +50,11 @@
 use bevy::asset::AssetApp;
 use bevy::asset::io::AssetSourceBuilder;
 use bevy::asset::io::embedded::EmbeddedAssetRegistry;
+#[cfg(not(target_arch = "wasm32"))]
 use bevy::asset::io::file::FileAssetReader;
 use bevy::prelude::*;
 
+#[cfg(not(target_arch = "wasm32"))]
 use crate::assets::user_dir::user_theme_dir;
 
 /// `AssetSourceId` of the user-themes asset source. Use it as
@@ -235,11 +237,16 @@ const CLASSIC_THEME_SVGS: &[(&str, &[u8])] = &[
 /// Returns the `&mut App` so the call can be chained from the binary
 /// entry point.
 pub fn register_theme_asset_sources(app: &mut App) -> &mut App {
-    let root = user_theme_dir();
-    app.register_asset_source(
-        USER_THEMES,
-        AssetSourceBuilder::new(move || Box::new(FileAssetReader::new(root.clone()))),
-    );
+    // User themes are stored on the filesystem; wasm32 has no filesystem and
+    // `FileAssetReader` is not available on that target.
+    #[cfg(not(target_arch = "wasm32"))]
+    {
+        let root = user_theme_dir();
+        app.register_asset_source(
+            USER_THEMES,
+            AssetSourceBuilder::new(move || Box::new(FileAssetReader::new(root.clone()))),
+        );
+    }
     app
 }