fix: Make the GPS serial port configurable (#82)

[cwill747]

Fixes #82. Split out of #87 — the contributed patch bundled both issues into one commit.

Problem

GPS was hardcoded to /dev/ttyS0, which is where the AIO V2 board exposes it on CM4-based uConsoles. CM5-based units expose the same UART at /dev/ttyAMA0, so GPS silently never worked there.

Fix

• Auto-detection now probes /dev/ttyS0 then /dev/ttyAMA0.
• The port can be pinned explicitly via Settings > GPS Device or the MESHCORE_GPS_DEVICE env var, which wins over both auto-detection and gpsd — needed when gpsd is running but bound to the wrong device, the case reported in the issue.
• An explicit device is accepted even if the node is absent at startup (e.g. a USB GPS plugged in later); start() surfaces the open error rather than falling back silently.
• Changing the setting rebuilds the provider in place, so it takes effect without restarting the app.

The GPS provider is now constructed after the settings load rather than before, since it needs the configured path.

Credit

Based on the patch contributed by @educationalpurposes in #81. Unmodified apart from the split, a MESHCORE_GPS_DEVICE entry in the AGENTS.md env var table, and a comment tidy.

Verification

uv run pytest: 101 passed, 1 skipped. ruff check / ruff format --check clean. Not tested on CM5 hardware — I don't have a unit; the reporter confirmed the change fixes it for them.

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