[backport/1.4] frontend: Discard tray MAVLink listeners on destroy

[joaoantoniocardoso]

This is a backport of #4046 into 1.4.

[github-actions]

Automated PR Review

0. Summary

• Verdict: LGTM

Backport of #4046 into 1.4-dev. Stores the HEARTBEAT listener in HealthTrayMenu.vue and the GLOBAL_POSITION_INT / GPS_RAW_INT|GPS2_RAW listeners in GpsTrayMenu.vue, wrapping each in markRaw (so Endpoint.latestData isn't deep-observed every frame) and calling .discard() in beforeDestroy to prevent listener stacking across remounts/HMR. Diff applies cleanly on 1.4-dev (Listener.discard() already exists there), and the only deviation from the master version — this.last_heartbeat_date = new Date() instead of autopilot_data.setLastHeartbeatDate(new Date()) — correctly matches the pre-centralization state of the 1.4 branch.

No further comments, nice job 👍

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