Add configurable tap zones, gestures, and keyboard mappings

[mokhov]

Problem

Fixed controls cannot accommodate different devices, handedness, accessibility needs, and established reader habits.

Proposed behavior

Provide an editor for screen zones, gestures, and keyboard shortcuts mapped to reader actions.

Acceptance criteria

• Readers can assign tap, double-tap, long-press, swipe, and supported keyboard inputs.
• The editor detects and explains conflicting assignments.
• Text selection, links, footnotes, annotations, and dialogs take priority over page-turn gestures.
• A safe default profile and one-action Reset are always available.
• Custom mappings never remove keyboard focus from interactive controls.