[FEAT] session ledger + one-click restore

[haimbj1]

What

• session_ledger.json: the collector records every session it sees and keeps dead ones 14 days (name, sessionId, cwd, tmux, kind, lastSeen). Unit-tested.
• The Sessions panel shows an "Ended sessions" section from the ledger. Tick rows, click Restore — the server resumes each with claude --resume <id> in its own tmux session. No model tokens.
• Restore skips live sessions, existing tmux names, and unknown ids; it reports each skip.

Why

A reboot kills every session and the registry entries with it. PR-session links drop and the developer must dig session ids out of transcripts by hand.