TASK-032 TUI: Quick-add shortcuts for task and scratch note

Description

Creating a Things task or scratch note currently requires opening the kk overlay, filtering to the entry, then dealing with an input prompt — 3 interactions. Add direct single-key shortcuts that jump straight to the input.

Proposed bindings:

• n → native KkInputScreen("new task", "task title…") → runs task <title> (Things 3 task creation)
• S (Shift+s) → native KkInputScreen("scratch", "what's on your mind?") → runs the existing _scratch_cmd()

These bypass the kk menu entirely. They use the existing KkInputScreen modal and run_command() worker.

Check that n and S don't conflict with any existing global bindings before landing.

Acceptance Criteria

• #1 n key opens a native text prompt and creates a Things task on submit
• #2 Shift+S opens a native text prompt and appends a timestamped scratch note
• #3 Both bindings work from any page
• #4 Empty submit is a no-op (no command run)
• #5 Confirmation toast on success
• #6 Bindings shown in Footer or help overlay