The table plane can be set: a tableOffsetMm setting and POST /settings/table-here from the arm's lowest point now #162

Part of #144.

Done:

• [ ] Software/so101/src/server.ts: a tableOffsetMm setting (-100..100, validated, persisted with the other settings) moves the virtual table. POST /settings/table-here (same-origin, admin only) sets it from the arm's lowest link right now, rounded so the resting arm is not flagged; the floor margin still applies above it.
• [ ] Test: after table-here, the resting pose has no violation; an out-of-range value is a 400.
• [ ] forge.json untouched. bun --cwd Software/so101 test passes.