fix: address critical thread leak and stream corruption bugs

[ImDevinC]

Summary

This PR addresses critical bugs identified in the Discord RPC plugin that could cause thread accumulation and socket stream corruption, along with several medium-priority improvements.

Critical Fixes

Thread Leak (asyncdiscord.py)

• Problem: Polling threads were not properly cleaned up on disconnect, causing zombie threads to accumulate with each reconnection
• Solution: Added thread tracking and proper join with 3s timeout in disconnect()
• Impact: Prevents memory leaks and thread exhaustion over time

Stream Corruption (sockets.py)

• Problem: Socket timeout during header read returned None instead of raising an error, causing the stream to become desynchronized
• Solution: Changed _recv_exact() to always raise EOFError on timeout, removed is_header parameter
• Impact: Eliminates mysterious disconnections from corrupted stream state

Medium Priority Fixes

Authentication State Checks (backend.py)

• Problem: API methods only checked connection state, not authentication state, leading to rejected RPC calls
• Solution: Added _is_authed checks to all 10 RPC API methods
• Impact: Prevents sending commands before authentication completes

Warning Logs Restored (backend.py)

• Problem: Silent failures made debugging difficult
• Solution: Added warning logs for all connection/auth failures
• Impact: Improved debuggability for users and developers

Connection State Check (asyncdiscord.py)

• Problem: is_connected() only checked polling flag, not actual socket state
• Solution: Now checks both self.polling and self.rpc.socket is not None
• Impact: Catches edge cases where socket died but thread hasn't detected it

Socket Timeout Reduction (constants.py)

• Problem: 2s timeout caused slow shutdown and compounded thread leak issue
• Solution: Reduced SOCKET_RECEIVE_TIMEOUT from 2s to 0.5s
• Impact: Faster shutdown and cleaner reconnects

Additional Improvements

• Add proper thread safety with _setup_lock for client initialization
• Remove dead code calling non-existent backend callback methods (DiscordCore.py)
• Add missing VOICE_SETTINGS_UPDATE event handler (bug fix)
• Simplify socket implementation from select-based to blocking with timeout
• Improved error messages and logging consistency

Testing Recommendations

1. Rapid connect/disconnect cycles (verify no thread accumulation)
2. Network interruption during send (verify stream recovery)
3. API calls before authentication (verify warning logs)
4. Clean shutdown timing (should complete in <1s)
5. Long-running connection stability

Breaking Changes

None - all changes are internal improvements and bug fixes.