feat: HTTP extension bridge for fingerprint browsers

[HuryDon]

Summary

• Replace WebSocket-only extension connectivity with HTTP-first bridge (hello / poll / callback) so Hubstudio / AdsPower fingerprint browsers can stay connected when local WS fails
• Keep /ws as compatibility fallback (EXT_TRANSPORT=auto|http|ws)
• Redefine health: recent hello/poll + flowKey, plus transport field (http | ws | none)
• Extension prefers HTTP; side panel / flow status show active transport

Key changes

• flow-agent/omniflash/http_bridge.py — session registry + command queue
• flow-agent/cli/api.py — POST /api/ext/hello, GET /api/ext/poll, enhanced /health
• flow-agent/omniflash/bridge.py — send_message enqueues over HTTP when session online
• flow-chrome-extension/* — HTTP transport + host_permissions for :8001
• Docs/config: EXT_TRANSPORT, EXT_SESSION_TTL_SEC, EXT_POLL_INTERVAL_MS

Test plan

• pytest tests/test_http_bridge.py tests/test_ext_http_api.py (9 passed)
• Local hello/poll/health roundtrip shows transport: http
• Official Chrome: load unpacked extension, open Flow, /health shows connected
• Hubstudio: HTTP path works without WS
• AdsPower: same as Hubstudio when local HTTP is reachable

Notes

Upstream write access is not available to this account; PR is opened from fork HuryDon/flow-agent.
Local main already contains a merge commit of this branch for local use.