chore: define analytics events for conversation funnel

[2sem]

Goal

Define the analytics events needed to measure Conversation Mode adoption and drop-off.

Scope

• Specify key events such as entry, mic tap, speech recognition success/failure, translation appended, speaker switch, dismiss, and restore/retry paths if applicable
• Keep event naming consistent and implementation-friendly
• Include the minimum useful properties needed for later analysis

Acceptance Criteria

• There is a clear event list for the conversation funnel
• The events are scoped tightly enough for implementation without ambiguity
• The plan helps measure activation, completion, and failure points

Why

2.3.0 needs a measurable hero-feature launch, not just a feature shipment.