[Tech-debt] Make Erst-Sync constants configurable & pause the re-gossip sweep when hidden

[ThePhantomPuppet]

GOSSIP_COOLDOWN_MS (30s), the periodic re-gossip sweep (60s), ROSTER_MAX (512)
and AVATAR_IMPORT_CAP (96KB) are hardcoded defaults from the initial build.

• Make them configurable.
• Pause the periodic sweep on document.hidden to save battery and wake-ups.