The page reconnects /live by itself within 1 s (backoff 0.25 to 2 s) behind a small 'reconnecting…' chip, never reloading #234

Split from #202, the browser half.

Done:

• [ ] When /live closes, Software/so101/src/ui/lobby-panel.js reconnects with backoff (0.25, 0.5, 1, then 2 s at most) and never reloads the page. The same session cookie means the same identity. A small chip says "reconnecting…" in a space reserved for it, so nothing shifts; it disappears on the first message. The page never says "simulated" while it waits.
• [ ] While disconnected, the page sends no motion and its drive controls are disabled.
• [ ] Tests (the existing ui test style, with a fake WebSocket): the retry delays follow the backoff, the chip shows and clears, and no move is sent while disconnected.
• [ ] forge.json is untouched. bun --cwd Software/so101 test passes, and the first-visit gzip budget still holds.