miner-app: guard fresh installs on disk space, refuse mismatched node/miner pairs

[adamtpang]

Overview

Two guards for failures that currently surface only as a silent stall or a generic crash. Both were reproduced on a real Windows 11 machine. Draft because I have no local Flutter toolchain, so CI here is the first compile check.

1. Free disk space before a fresh node install

A fresh install downloads the node and then syncs roughly 100 GB of chain data into ~/.quantus/node_data. MINING.md states 100 GB minimum, 500 GB recommended. The setup screen had no free-space check, so a machine with too little room downloads successfully and then fails partway through sync with no explanation. Observed on a machine with 27 GB free.

• DiskSpaceService.freeBytesForPath probes the volume holding a path: PowerShell on Windows, df -kP elsewhere. Returns null rather than throwing, so a failed probe never blocks setup on its own.
• MinerConfig gains bytesPerGb, minNodeDiskBytes, recommendedNodeDiskBytes, sourced from MINING.md.
• The check runs inside the fresh-install branch only and throws before fetching the node version. It surfaces through the existing catch, so no new UI.

The update path is deliberately not gated. A returning user's binary update is a small download onto a volume that already holds their chain data, and blocking a protocol or security update over disk space would be wrong.

2. Mismatched node and miner auth pair

Node v0.10.0 requires miner QUIC auth; miner releases before v4.0.0 do not support it. The app checks each binary for updates independently and does not gate Start Mining on the pair matching. A user whose miner is older than their node, or whose update was interrupted, can start mining and see only "Miner died during startup".

Reproduced: miner-cli 2.1.2 beside quantus-node 0.10.0, which the official mining script's own probe classifies as "Mixed: stop".

• PairCompatibilityService.check runs the same --help probe the script runs, against the installed binaries. Returns null if either probe fails, so a broken or wrong-architecture binary stays with the existing "binary not found" path.
• _startMiner refuses a mixed pair with a snackbar naming both versions and which one to update. Compatible or unknown pairs proceed unchanged.

Validation

Both services follow the existing GpuDetectionService shape: static, Process.run, tagged logger, null on probe failure. Lines are within the 120-character format limit, and the diff conforms to the repo's prefer_single_quotes and always_declare_return_types rules. Not yet compiled locally, relying on CI flutter analyze.