Phone watchers see the last 3 chat lines faintly over the bottom of the video, never while driving #288

From #51 (TikTok/Twitch Live style).

Done:

• [ ] In phone portrait, someone who is watching (not driving) sees the last 3 chat lines as a transparent overlay along the bottom of the video. The lines fade after a few seconds, and a tap on the video hides them.
• [ ] The overlay is never shown to the current driver, and it is never shown in a way that covers the model.
• [ ] Lines are text only (escaped). A test: watcher sees 3, driver sees 0.
• [ ] forge.json untouched. bun --cwd Software/so101 test passes.