fix(wasm): gate wasm32-only imports behind cfg, add binaryen wasm-opt pass

- Gate `Startup` and `user_theme_dir` imports in theme/registry.rs
  behind `#[cfg(not(target_arch = "wasm32"))]` — they are only used
  in the non-wasm code path, eliminating two unused-import warnings
  in the WASM release build.
- Rebuild canvas_bg.wasm and solitaire_wasm_bg.wasm with wasm-opt -Oz
  (binaryen v129); canvas_bg.wasm drops from 57 MB → 30 MB.
- Add solitaire_web/Cargo.toml stub to server Dockerfile so
  `cargo fetch --locked` resolves all workspace members.

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

solitaire_engine/src/theme/registry.rs

@@ -22,11 +22,15 @@
 use std::path::Path;
 
 use bevy::log::warn;
-use bevy::prelude::{App, Plugin, Resource, Startup};
+use bevy::prelude::{App, Plugin, Resource};
+#[cfg(not(target_arch = "wasm32"))]
+use bevy::prelude::Startup;
 use serde::Deserialize;
 
 use super::ThemeMeta;
-use crate::assets::{DARK_THEME_MANIFEST_URL, user_theme_dir};
+use crate::assets::DARK_THEME_MANIFEST_URL;
+#[cfg(not(target_arch = "wasm32"))]
+use crate::assets::user_theme_dir;
 
 /// One entry in the [`ThemeRegistry`] — the data the picker UI needs
 /// to render a row and load the theme on selection.