Offer Pipeline

Adds a Stats screen: stage funnel, response rate, applied-to-offer rate and offer CTC range. Saved data is unchanged.

Offers are locked: editing one asks for a reason, kept in an edit log. Offers now show in Follow-ups. Existing applications are upgraded automatically.

Adds expected CTC, location, work type, a follow-up date with a "Follow-ups due" list, and stage history. Existing applications are upgraded automatically.

First version: track job applications by stage, with counts per stage.