fix(web): resolve wasm32 runtime panics; game boots and renders in Firefox

Fixes found while testing the Bevy WASM build in a real browser:

1. chrono wasmbind: add `wasmbind` feature to workspace chrono dep so
   Local::now()/Utc::now() use js-sys::Date on wasm32 (previously
   fell through to std::time::SystemTime which panics on wasm32).

2. std::time::SystemTime: replace all remaining direct SystemTime::now()
   calls (4 sites across game_plugin, difficulty_plugin, time_attack_plugin,
   solitaire_data/storage) with chrono::Utc::now() which is wasm32-safe.

3. user_dir: return empty PathBuf (instead of panicking) when data_dir()
   is None on wasm32; there is no filesystem in the browser so user themes
   are unsupported and a benign empty path is correct.

4. ThemeRegistryPlugin: gate build_registry_on_startup to non-wasm32
   (the filesystem scan for user themes has nothing to scan in the browser;
   only the bundled embedded themes are available).

5. AssetMetaCheck::Never: configure AssetPlugin in solitaire_web to skip
   `.meta` sidecar fetches — we don't ship .meta files, so the default
   AssetMetaCheck::Always produced a 404 flood on every card/background asset.

Result: `http://localhost:<port>/play` boots in Firefox with zero errors
and renders the full Bevy game — home screen, onboarding modal, HUD all
visible. Assets load correctly from /assets/. Chromium has a separate
wgpu-27/ANGLE/GLES shader translation bug (not in our code); Firefox works.

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

solitaire_engine/src/game_plugin.rs

@@ -7,12 +7,12 @@
 //! file is deleted so the next launch starts fresh.
 
 use std::path::PathBuf;
-use std::time::{SystemTime, UNIX_EPOCH};
+
+use chrono::Utc;
 
 use bevy::prelude::*;
 use bevy::tasks::{AsyncComputeTaskPool, Task, futures_lite::future};
 use bevy::window::AppLifecycle;
-use chrono::Utc;
 use solitaire_core::game_state::{DrawMode, GameMode, GameState};
 use klondike::KlondikePile;
 use solitaire_core::solver::{SolverConfig, SolverResult, try_solve};
@@ -312,9 +312,7 @@ fn tick_elapsed_time(
 }
 
 fn seed_from_system_time() -> u64 {
-    SystemTime::now()
-        .duration_since(UNIX_EPOCH)
-        .map_or(0, |d| d.as_nanos() as u64)
+    Utc::now().timestamp_nanos_opt().unwrap_or(0) as u64
 }
 
 /// Walks forward from `initial_seed` (incrementing by 1 with wrapping