Migrate off deprecated CannedMessageConfig.allow_input_source (removed field, stop showing/writing)

[garthvh]

🤖 Good task for an agent: this is a self-contained, well-scoped migration ideal for an automated coding agent.

Summary

The Apple app still shows and writes the deprecated CannedMessageConfig.allow_input_source field. Per the upstream deprecated-fields audit, this field has been removed from active use — there is no successor field; it should simply no longer be shown or written. Apple is marked ❌ Still shown and writable (Android is in the same state).

Field	Detail
Proto message	CannedMessageConfig
Deprecated field	allow_input_source = 10
Deprecated since	v2.7.4 (2025-08-09 pre-release)
Stable	v2.7.15
Replacement	None — removed from active use
Apple status	❌ Still shown and writable

Audit reference

https://github.com/meshtastic/design/blob/master/standards/audits/deprecated-fields-audit.md

Suggested approach

1. Grep the Apple codebase for the Swift symbol for this field (e.g. allowInputSource, cannedMessageConfig.allowInputSource, and the Canned Messages settings form).
2. Remove the control (text field / picker) that exposes allow_input_source from the Canned Messages settings UI.
3. Stop writing allowInputSource when saving CannedMessageConfig to the device.
4. Keep backward-compat on read: tolerate the field still arriving from older firmware without crashing; just don't surface or persist it.
5. Verify the Canned Messages settings screen no longer shows the control and that saving config no longer sets this field.