Make Enter honor the configured message action

[trufae]

Pressing Enter on a message always started a reply, ignoring the configured double-click action. This addresses the first follow-up in #57.

Return and keypad Enter now share the same action dispatcher as double-click: Reply, either reaction, Show Contact Details, Show Message Menu, or Do Nothing. Keyboard menus are anchored to the focused message. Only Reply moves focus to the composer, so reactions and Do Nothing preserve keyboard navigation.

Settings, shortcut help, the README and the accessibility checklist describe the behavior. Modified Enter, embedded controls, selection mode and unaccepted contact requests retain their existing handling.

Validation:

• Native Linux builds pass with warnings treated as errors, with both a11y=true and a11y=false (webxdc=false locally).
• All 14 existing Meson test suites pass in both configurations.
• git diff --check passes.
• Interactive keyboard and screen-reader verification remains pending: the execution environment cannot start Xvfb's display sockets. The checklist covers every action, keypad Enter, modifiers, filtering and focus.

Based directly on main. This PR addresses the Enter-action item and leaves the broader #57 open.