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_server/Dockerfile

@@ -21,11 +21,12 @@ COPY solitaire_server/Cargo.toml   ./solitaire_server/Cargo.toml
 COPY solitaire_app/Cargo.toml      ./solitaire_app/Cargo.toml
 COPY solitaire_assetgen/Cargo.toml ./solitaire_assetgen/Cargo.toml
 COPY solitaire_wasm/Cargo.toml     ./solitaire_wasm/Cargo.toml
+COPY solitaire_web/Cargo.toml      ./solitaire_web/Cargo.toml
 
 # Stub every workspace crate so `cargo fetch --locked` resolves the full
 # dependency graph without requiring source files beyond Cargo.toml.
 RUN for crate in solitaire_core solitaire_sync solitaire_data solitaire_engine \
-        solitaire_server solitaire_app solitaire_assetgen solitaire_wasm; do \
+        solitaire_server solitaire_app solitaire_assetgen solitaire_wasm solitaire_web; do \
       mkdir -p $crate/src && echo "pub fn _stub() {}" > $crate/src/lib.rs; \
     done && \
     echo "fn main() {}" > solitaire_server/src/main.rs && \