fix: WebSocket connection leak + reuse existing SHA1/base64 libs
- Fix MCP connection leak: null transport/client on close so
isConnected() returns false, preventing duplicate connections
- Remove 106 lines of reimplemented SHA1/base64, use existing:
- sha1/sha1.h from examples/gguf-hash/deps
- base64.hpp from common/
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>