Standardize TUI model updates on KeyPressMsg and constructor reuse - #30

Summary

TUI architecture cleanup for internal/model, replacing tea.KeyMsg with tea.KeyPressMsg, making all Update() methods return copies, and routing entity creation through book.NewShelf/book.NewCollection.

What's new

• Shared key handler - handleCommonKeys in internal/model/tea.go centralizes ctrl+c/esc/ctrl+p for all six TUI Update methods.
• Immutable get* models - getMarkModel and getCollectionModel sync derived state via value-receiver reloadFromForm(); View() no longer triggers side effects.
• Immutable edit* models - editShelfModel, editCollectionModel, and editMarkModel copy on form completion and wire entities before the write command runs.
• Constructor reuse in the TUI - editShelfModel and editCollectionModel create v2 shelves/collections through book.NewShelf/book.NewCollection instead of inline ID/timestamp assignments.
• Terminal error plumbing - new ErrorProvider interface lets runProgram return TUI errors after exit; ResultView suppresses output on error.

Behavior changes / guardrails

• Derived model state starts nil and form constructors create local scaffolding structs (for example &book.Shelf{}) rather than storing fake values in the model.
• updateShelfFileCmd closures now only add/touch entities and write files; creation happens in Update().
• Shelf.ValidateNewCollectionName rejects empty or duplicate collection names in the TUI, matching the CLI behavior.
• getShelfModel.View now displays the actual picked shelf name instead of the "fake" debug placeholder.
• markModel.verifyMark is now a value receiver, matching verifyCollection.

Testing

• make check runs fmt, vet, lint, and tests; all green.