Design replacement for Global Variable readouts

[burzl89]

Context

The optional Global Variable readouts were removed because enabling them can cause widget errors when they are displayed.

Goal

Find a safer replacement for user-configurable extra telemetry/readout slots without directly using the previous GV display path.

Possible directions

• Add generic custom telemetry slots backed by normal EdgeTX sensors.
• Add a fixed optional sensor pair instead of model global variables.
• Investigate whether model.getGlobalVariable() needs guards, availability checks, or different EdgeTX/LVGL update timing.
• Keep the LINK card full-width unless the replacement is known to be stable.

Acceptance criteria

• No widget crash when optional readouts are enabled.
• Missing/unsupported sources render as ---.
• Option changes rebuild the UI cleanly.
• Behavior is documented in the README.