feat(audio): 오디오 시스템 추가

- just_audio 패키지 추가
- AudioService 구현 (BGM/SFX 재생)
- assets/audio/bgm/, assets/audio/sfx/ 에셋 추가

pubspec.lock

@@ -9,6 +9,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.13.0"
+  audio_session:
+    dependency: transitive
+    description:
+      name: audio_session
+      sha256: "2b7fff16a552486d078bfc09a8cde19f426dc6d6329262b684182597bec5b1ac"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.1.25"
   boolean_selector:
     dependency: transitive
     description:
@@ -41,6 +49,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.19.1"
+  crypto:
+    dependency: transitive
+    description:
+      name: crypto
+      sha256: c8ea0233063ba03258fbcf2ca4d6dadfefe14f02fab57702265467a19f27fadf
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.0.7"
   cupertino_icons:
     dependency: "direct main"
     description:
@@ -73,6 +89,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "7.0.1"
+  fixnum:
+    dependency: transitive
+    description:
+      name: fixnum
+      sha256: b6dc7065e46c974bc7c5f143080a6764ec7a4be6da1285ececdc37be96de53be
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.1.1"
   flutter:
     dependency: "direct main"
     description: flutter
@@ -109,6 +133,30 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "0.20.2"
+  just_audio:
+    dependency: "direct main"
+    description:
+      name: just_audio
+      sha256: f978d5b4ccea08f267dae0232ec5405c1b05d3f3cd63f82097ea46c015d5c09e
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.9.46"
+  just_audio_platform_interface:
+    dependency: transitive
+    description:
+      name: just_audio_platform_interface
+      sha256: "2532c8d6702528824445921c5ff10548b518b13f808c2e34c2fd54793b999a6a"
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.6.0"
+  just_audio_web:
+    dependency: transitive
+    description:
+      name: just_audio_web
+      sha256: "6ba8a2a7e87d57d32f0f7b42856ade3d6a9fbe0f1a11fabae0a4f00bb73f0663"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.4.16"
   leak_tracker:
     dependency: transitive
     description:
@@ -237,6 +285,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.1.8"
+  rxdart:
+    dependency: transitive
+    description:
+      name: rxdart
+      sha256: "5c3004a4a8dbb94bd4bf5412a4def4acdaa12e12f269737a5751369e12d1a962"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.28.0"
   shared_preferences:
     dependency: "direct main"
     description:
@@ -330,6 +386,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.4.1"
+  synchronized:
+    dependency: transitive
+    description:
+      name: synchronized
+      sha256: c254ade258ec8282947a0acbbc90b9575b4f19673533ee46f2f6e9b3aeefd7c0
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.4.0"
   term_glyph:
     dependency: transitive
     description:
@@ -346,6 +410,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "0.7.6"
+  typed_data:
+    dependency: transitive
+    description:
+      name: typed_data
+      sha256: f9049c039ebfeb4cf7a7104a675823cd72dba8297f264b6637062516699fa006
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.4.0"
+  uuid:
+    dependency: transitive
+    description:
+      name: uuid
+      sha256: a11b666489b1954e01d992f3d601b1804a33937b5a8fe677bd26b8a9f96f96e8
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.5.2"
   vector_math:
     dependency: transitive
     description: