P2poolv2 process management

[R27-pixel]

Summary

Adds start/stop/restart process management for the p2poolv2 node, mirroring the Bitcoin Core process management from #.

What's included

• New p2poolv2_path setting (falls back to PATH lookup when unset, same as bitcoind_path)
• p2pool_process_manager on App reusing the existing generic ProcessManager, with start/stop/restart/poll/shutdown
• Spawns the node with --config <p2pool_conf_path>, matching the real systemd unit's ExecStart
• New "Process" tab on P2Pool Status with [s] Start, [t] Stop, [r] Restart
• New "P2Poolv2 executable" field in Settings
• p2poolv2 now shuts down gracefully alongside Bitcoin Core on quit

Testing

• Unit tests for command construction, missing-executable failure, idle-stop no-op
• Unit tests for the new Settings field and Process tab key handling
• Render test for the Process tab

[codecov]

Codecov Report

❌ Patch coverage is 80.22012% with 647 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/app.rs	53.06%	237 Missing ⚠️
src/components/bitcoin_status_view.rs	57.85%	169 Missing ⚠️
src/main.rs	61.84%	87 Missing ⚠️
src/process_manager.rs	76.47%	56 Missing ⚠️
src/bitcoin_logs.rs	83.81%	39 Missing ⚠️
src/components/bitcoin_client.rs	96.03%	20 Missing ⚠️
src/components/p2pool_websocket.rs	95.97%	14 Missing ⚠️
src/components/p2pool_client.rs	95.42%	8 Missing ⚠️
src/config.rs	61.90%	8 Missing ⚠️
src/components/p2pool_status_view.rs	98.71%	6 Missing ⚠️
... and 1 more

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