Add Enter key to exit TUI after render completes

[tjazerzen]

Summary

• Add Enter keybinding to exit the TUI, only active after rendering has finished
• Update CustomFooter to dynamically switch footer text to render-finished keybindings (enter: exit * ctrl+c: copy * ctrl+l: toggle logs) once rendering completes
• Track render completion state via _render_finished flag to guard the Enter exit action

Test plan

• Verify pressing Enter during rendering does nothing
• Verify pressing Enter after render completes exits the TUI
• Verify footer text updates from default keybindings to render-finished keybindings upon completion
• Verify Ctrl+D still works as a quit shortcut at any time

Visual changes

(note "enter to exit" next to "rendering finished" and "enter: exit" at the bottom).