feat: complete client-server architecture refactoring

Server:
- Split into views, routes, helpers, models modules
- Merged /ws/talk and /ws/update into single /ws/chat endpoint
- Replaced polling with push-based broadcast model
- Added username uniqueness validation on connect
- Fixed run_server arguments bug (workers parameter)
- Removed deprecated loop argument from Sanic listeners
- Replaced datetime.utcnow() with timezone-aware datetime.now(timezone.utc)

Client:
- Rewrote client as single-file module
- Migrated from websocket-client to websockets (asyncio)
- Fixed websocket-client conflict with asyncio event loop on Windows
- Added progress indicators for key generation, exchange, connection
- Added animated 3D spinning cube in UI
- Updated RSA key from 512 to 2048 bits

CLI:
- Removed unnecessary asyncio.run() wrapper
- Simplified entry point

cmd_chat/server/routes.py

@@ -0,0 +1,21 @@
+from sanic import Sanic, Request, Websocket
+
+from . import views
+
+
+def register_routes(app: Sanic) -> None:
+    @app.route("/get_key", methods=["GET", "POST"])
+    async def get_key_route(request: Request):
+        return await views.get_key(request, app)
+
+    @app.websocket("/ws/chat")
+    async def chat_ws_route(request: Request, ws: Websocket):
+        await views.chat_ws(request, ws, app)
+
+    @app.get("/health")
+    async def health_route(request: Request):
+        return await views.health(request, app)
+
+    @app.delete("/clear")
+    async def clear_route(request: Request):
+        return await views.clear_messages(request, app)