Avoid saving cancelled task edits

[oiahoon]

Summary

• distinguish task edit continuation, save, and cancel actions
• return to normal mode without saving when the edit is cancelled
• clear a pending o/O insertion position when its edit is cancelled
• cover Escape and Enter behavior with focused unit tests

Why

Task editing previously returned the same boolean for both Enter and Escape. The caller treated either result as a completed save, so pressing i followed by Escape created a blank task. This addresses the task-edit cancellation behavior described in #32 without changing recurrence generation.

Validation

• cargo test --locked (7 passed)
• cargo check --locked --all-targets --all-features
• cargo clippy --locked --all-targets --all-features
• cargo fmt -- --check
• manual TUI: i then Escape leaves no data file or task
• manual TUI: o then Escape followed by a normal insert creates one task at order 1