Sessions not properly marked as inactive when CLI exits #10

Problem

Sessions stay active: true when CLI dies unexpectedly. Users see "session revival" messages for dead sessions.

Current Modeling Issue

We only track active: true/false but this doesn't distinguish between:

• Offline - CLI not responding (laptop closed, network down) but may reconnect
• Terminated - CLI process exited, won't come back

The active flag should represent whether the process literally exited, not just connectivity.

UX Impact

• Dead sessions appear alive for up to 10 minutes
• "Session revival" messages when there's nothing to revive
• Can't tell if session is temporarily offline or permanently dead

Code Pointers

• CLI cleanup: happy-cli/src/start.ts:274, 330 (doesn't run on crash)
• Server session-end: happy-server/sources/app/api.ts:667-714
• Server timeout: happy-server/sources/app/timeout.ts:15 (10 min delay)