docs: add settings validation reference

[Copilot]

Comprehensive reference documenting the validation rules enforced on every radio config and module config settings screen, sourced from both protobuf max_size annotations and the UI form layer.

• Device config screens (User, Device, Position, Power, Network, Display, LoRa, Bluetooth, Security) — per-field type, constraint, and conditional visibility rules
• Module config screens (MQTT, Serial, Ext Notification, Store & Forward, Range Test, Telemetry, Canned Message, Audio, Remote Hardware, Neighbor Info, Ambient Lighting, Detection Sensor, Paxcounter, Status Message, Traffic Management, TAK) — same treatment
• Channel config — name byte limit, PSK length constraints (0/16/32 bytes)
• Interval configurations — all 17 IntervalConfiguration contexts with their exact allowed value sets in seconds
• UI component validation — documents how each reusable core:ui component (EditTextPreference, DropDownPreference, EditIPv4Preference, EditBase64Preference, BitwisePreference, SliderPreference, etc.) enforces constraints

Example entry:

Field	Type	Validation	Notes
long_name	String	maxSize: 39 bytes (proto max_size: 40)	Required (non-blank)
latitude	Double	Range: −90.0 to +90.0	Visible only when fixed_position = true
fixed_pin	String	Exactly 6 digits	—

Also notes known discrepancies (e.g. allow_input_source UI enforces 63 bytes while proto specifies max_size: 16).

[Copilot]

Pull request overview

Adds a comprehensive documentation reference for validation rules across radio/device/module/channel configuration screens, including interval presets and reusable core:ui component behavior, with constraints sourced from protobuf max_size annotations and the settings UI form layer.

Changes:

• Introduces docs/settings-validation.md as a field-by-field validation reference for config screens.
• Documents IntervalConfiguration allowed value sets and shared constants (e.g., GPIO pin ranges).
• Summarizes validation/enforcement behavior of reusable core:ui preference components.