Fix sync race conditions and prevent endless loops - #63

Synchronization logic had three critical issues causing incorrect "world removed from remote" alerts during first-time uploads and potential infinite sync loops.

Changes

• Prevent endless sync loop: world_store_sync_check while loop used static world_list_merged reference. After syncing, the same world object could match the condition again since its properties weren't mutated. Now explicitly marks worlds as synced by setting local = remote = Math_max(local, remote) before next iteration.

• Fix database rename race: During first-time world upload, chunks_rename ran in parallel with data upload. If upload failed mid-flight, IndexedDB could have new ID while state retained old ID. Changed to await chunks_rename() before upload starts.

• Enable error recovery: Re-enabled commented code that resets mod_r = WORLD_STORED_NOT when world missing from remote, allowing retry. Removed production debugger statement.

// Before: could loop infinitely
while (world_syncable = world_list_merged.find(world => 
    world.local !== world.remote
)) {
    await world_store_sync(world_syncable);
    // world_syncable.local/remote unchanged, matches again
}

// After: marks synced
while (world_syncable = world_list_merged.find(world => 
    world.local !== world.remote
)) {
    await world_store_sync(world_syncable);
    const synced = Math_max(world_syncable.local, world_syncable.remote);
    world_syncable.local = world_syncable.remote = synced;
}

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