docs: RetroArch frontend launch race, claim-based online status and TZ

[gantoine]

Docs follow-ups from the 5.4.0-alpha.2 save sync and Install on device testing. Companion to rommapp/romm#5190.

• RetroArch Cloud Sync: a new "Known RetroArch issues" section.
  • Launching from a frontend: muOS starts RetroArch with a game, and the core loads the local save before the startup sync finishes. The stale save then goes up as the newest version. The section covers the workaround (open RetroArch first and let it sync) and recovery (delete the bad version from the autosave history).
  • Begin failed / HTTP -1 after Close Content: the request never reaches RomM, and restarting RetroArch fixes it.
• Devices: the online status now also counts a device that claimed installs in the last 90 seconds (fix(devices): count a device that polls claim as online romm#5190), so apps that poll show as online.
• Saves & States: version names from devices and RetroArch use the server's time, which follows TZ. env.template gains TZ in fix(devices): count a device that polls claim as online romm#5190, and the generated env-vars table picks it up once that merges.

The new section sits apart from the Troubleshooting list so it doesn't conflict with #172 or #173.

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