Follows PLAN 4.2: create .ykanban, take the lock, then build the database.
SqliteDatabase.Create writes every migration and the preset columns in a
single transaction and deletes the file again on failure, so no empty
version-0 database is left behind. WorkspaceSession.Initialize keeps the lock
into the session instead of releasing and re-acquiring it.
A lock conflict while initializing shows the lock-conflict page.
- board: columns in id order, per-column sort via config, column width presets (240/320/480) and custom 160-720
- column header new-card and edit/delete menu; card menu delete and move-to-column
- card editor as one unit of work with dirty check and discard confirmation; nested tag picker writes new tags immediately
- schema constraint violations mapped to typed exceptions and shown inside the modal
- dialog service with owner stack so nested modals center on their parent and get FlowDirection
- repository: filtered GetCards, CountCards; view-model tests