cloud-sync: dynamic directory chooser in the config

[maxengel]

Summary

Let users pick sync directories (BACKUPPATH / RESTOREPATH / SYNCPATH, etc.) dynamically instead of the hardcoded /storage/roms defaults.

Scope

• A chooser (EmulationStation UI or a guided config flow) that validates and persists paths into cloud_sync.conf.
• Account for the NFS overlay (/storage/games-external → /storage/roms) and keep the rules file consistent with the chosen paths.

Related: system-backup revamp.

[maxengel]

Scope guardrail: must stay within the cloud-sync allowlist (saves/savestates/screenshots + system-backup zip) and never risk non-synced local data (ROMs/BIOS/art). Preserve excludes in any sync/bisync direction; dir chooser stays within save dirs; keep system-backup partitioned from the saves flow.