[Event sessions] Event profile preparation and channel snapshot recovery

[RCGV1]

Parent design: meshtastic/design#120 (event session addendum)
Firmware capability: meshtastic/firmware#11109

Scope

Provide Web-client parity for event sessions where the browser can manage a connected radio:

• Resolve event metadata and show the event profile/change summary.
• Capture a protected, exportable channels-only snapshot before the user applies an event profile.
• Apply and verify the firmware capability's channel-profile operation, then offer verified recovery once the radio is back on standard firmware.
• Do not claim browser OTA or background venue-arrival support where the browser/platform cannot provide it. Link to the appropriate supported updater instead.

Requirements

• Default is replace, with add only as an explicit alternate mode.
• Keep channel keys out of ordinary browser storage, telemetry, URLs, and logs. Clearly describe the export/encryption boundary.
• Bind recovery to the same device identity and snapshot digest; refuse a mismatched or incomplete restore.
• Surface firmware/profile incompatibility and reconnect/readback failures rather than treating a disconnect as completion.

Verification

• Tests for profile validation, identity/digest matching, protected snapshot lifecycle, and recoverable error states.
• Browser integration test with a mock radio session for prepare/apply/reconnect/restore.

Related: Meshtastic-Android#3911, meshtastic/design#36.