Use git pull to update pedalboards

[sastraxi]

Replaces the "Update sample pedalboards" flow (which ran a script that overwrote the sample pedalboards from a fixed remote) with "Sync pedalboards", which pulls/merges from a user-configured git remote. This config value lives in default_config.yml, e.g.

pedalboards: https://github.com/TreeFallSound/pi-stomp-pedalboards

Local edits are preserved across syncs, to the extent git allows it. Update status (including conflicts) show up on the LCD. This includes:

• "Up to date"
• "N update(s) applied"
• "Sync aborted: conflicts" (with conflicting filenames listed)
• "Sync failed: no network"
• "Sync error — see logs: journalctl -u mod-ala-pi-stomp"
• "Pedalboards cloned — restart sound engine to load" (first-time clone)
• "Remote mismatch: local commits not on origin. Resolve via SSH then sync."

Persistent notification icon

A new orange alert icon appears on the LCD whenever the last sync attempt left a problem (conflicts, network error, remote mismatch, etc.) — so the user knows their pedalboards are out of sync without re-opening the menu. It clears automatically once a sync succeeds.

First-boot behavior

If ~/.pedalboards is missing/empty and a remote is configured, pi-stomp clones it automatically at startup rather than requiring the user to SSH in. This supplements the behaviour provided by pi-gen-pistomp et al.