fix(calibration): add explicit "Save port" button so ports can be updated without recalibrating (#53)

[pgsharma]

Problem

Fixes #53.

The SO-101 uses serial, so its port (/dev/tty.usbmodem…, COM…) changes when the arm is unplugged/reconnected. As the issue reports, the only visible way to update a robot's saved port was to redo the entire calibration.

Root cause

The port already does autosave: persistPort() in Calibration.tsx writes the port to the robot record on input blur and on port-detection (added in 027575b, ~2 weeks before this issue was filed). But it's silent — no button, no confirmation — so users don't realize the change was saved and reasonably assume recalibration is the only path. Hence "Missing save button during calibration."

Fix (frontend only)

• Add a visible Save button next to the Port field that persists the port to the robot record and shows a confirmation toast (Saved <port> for the leader/follower — no recalibration needed).
• Add a one-line hint under the field noting ports change on reconnect.
• persistPort() now returns "saved" | "unchanged" | "skipped" | "error" (and checks res.ok) so the button can report success/failure. The existing silent onBlur / on-detect autosave is unchanged.

No backend change — POST /robots/{name} already does partial (port-only) merges via save_robot_record.

Testing

• npm run build ✓ and eslint src/pages/Calibration.tsx ✓ (clean).
• Manual: open Calibration for a robot → change Port → Save → toast confirms, record updates, no recalibration needed; teleop/record then use the new port.

frontend/dist/ intentionally not committed — CI (build_frontend.yml) rebuilds it on merge.

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