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_data/src/storage.rs

@@ -6,7 +6,7 @@
 use std::fs;
 use std::io;
 use std::path::{Path, PathBuf};
-use std::time::{SystemTime, UNIX_EPOCH};
+use chrono::Utc;
 
 use serde::{Deserialize, Serialize};
 use solitaire_core::game_state::{GAME_STATE_SCHEMA_VERSION, GameState};
@@ -234,9 +234,7 @@ pub fn load_time_attack_session_from_at(
 /// See [`load_time_attack_session_from_at`] for the rules under which
 /// the call returns `None` (missing file, corrupt JSON, expired window).
 pub fn load_time_attack_session_from(path: &Path) -> Option<TimeAttackSession> {
-    let now = SystemTime::now()
-        .duration_since(UNIX_EPOCH)
-        .map_or(0, |d| d.as_secs());
+    let now = Utc::now().timestamp().max(0) as u64;
     load_time_attack_session_from_at(path, now)
 }
 
@@ -254,9 +252,7 @@ pub fn delete_time_attack_session_at(path: &Path) -> io::Result<()> {
 /// current wall-clock time. Equivalent to constructing the struct
 /// manually and setting `saved_at_unix_secs` to `SystemTime::now()`.
 pub fn time_attack_session_with_now(remaining_secs: f32, wins: u32) -> TimeAttackSession {
-    let now = SystemTime::now()
-        .duration_since(UNIX_EPOCH)
-        .map_or(0, |d| d.as_secs());
+    let now = Utc::now().timestamp().max(0) as u64;
     TimeAttackSession {
         remaining_secs,
         wins,