feat(engine): art pass — PNG assets, custom font, and keyring v4 upgrade

Art pass (Phase 4):
- Generate placeholder PNG assets: face.png, back_0–4.png, bg_0–4.png via
  solitaire_assetgen gen_art binary (16×16 RGBA, embedded via include_bytes!)
- Add FiraMono-Medium font (assets/fonts/main.ttf) embedded at compile time
- Add FontPlugin: loads font at startup, exposes FontResource; gracefully
  falls back to default handle when Assets<Font> absent (MinimalPlugins tests)
- Wire CardImageSet into card_plugin: face/back PNGs replace solid-colour
  sprites when available; tests continue using colour fallback via MinimalPlugins
- Wire BackgroundImageSet into table_plugin: bg PNGs replace solid-colour
  background; empty set inserted when Assets<Image> absent in tests
- Fix hint highlight system (input_plugin): tint sprite.color directly instead
  of replacing the whole Sprite (which would discard the image handle)
- Export FontPlugin, FontResource, CardImageSet from solitaire_engine::lib
- Register FontPlugin in solitaire_app before other plugins

Dependency upgrades (latest releases):
- keyring "2" → keyring "4" + keyring-core "1" (v4 split architecture into
  separate core library crate)
- auth_tokens.rs: Entry::new now returns Result; delete_password →
  delete_credential; NoDefaultStore error variant handled
- solitaire_app: add keyring::use_native_store(true) at startup for Linux
  Secret Service / macOS Keychain / Windows Credential Store selection

ARCHITECTURE.md: fix Edition 2025→2021, update asset pipeline section,
add FontPlugin/CardImageSet/BackgroundImageSet to plugin and resource tables,
update Section 14 to reflect actual include_bytes!() rendering approach,
add Decision Log entries for embedded PNG and font decisions

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

solitaire_app/Cargo.toml

@@ -12,3 +12,4 @@ path = "src/main.rs"
 bevy             = { workspace = true }
 solitaire_engine = { workspace = true }
 solitaire_data   = { workspace = true }
+keyring          = { workspace = true }

solitaire_app/src/main.rs

@@ -3,12 +3,24 @@ use solitaire_data::{load_settings_from, provider_for_backend, settings_file_pat
 use solitaire_engine::{
     AchievementPlugin, AnimationPlugin, AudioPlugin, AutoCompletePlugin, CardAnimationPlugin,
     CardPlugin, ChallengePlugin, CursorPlugin, DailyChallengePlugin, FeedbackAnimPlugin,
-    GamePlugin, HelpPlugin, HomePlugin, HudPlugin, InputPlugin, LeaderboardPlugin,
+    FontPlugin, GamePlugin, HelpPlugin, HomePlugin, HudPlugin, InputPlugin, LeaderboardPlugin,
     OnboardingPlugin, PausePlugin, ProfilePlugin, ProgressPlugin, SelectionPlugin, SettingsPlugin,
     StatsPlugin, SyncPlugin, TablePlugin, TimeAttackPlugin, WeeklyGoalsPlugin, WinSummaryPlugin,
 };
 
 fn main() {
+    // Initialise the platform keyring store before any token operations.
+    // On Linux this uses the Secret Service (GNOME Keyring / KWallet); on
+    // macOS it uses the Keychain; on Windows it uses the Credential store.
+    // If the platform has no OS keyring (e.g. a headless CI box), keyring
+    // operations will fail gracefully with TokenError::KeychainUnavailable.
+    if let Err(e) = keyring::use_native_store(true) {
+        eprintln!(
+            "warn: could not initialise OS keyring ({e}); \
+             server sync login will be unavailable"
+        );
+    }
+
     // Load settings before building the app so we can construct the right
     // sync provider. Falls back to defaults if no settings file exists yet.
     let settings: Settings = settings_file_path()
@@ -32,6 +44,7 @@ fn main() {
                 ..default()
             }),
         )
+        .add_plugins(FontPlugin)
         .add_plugins(GamePlugin)
         .add_plugins(TablePlugin)
         .add_plugins(CardPlugin)