TUI: wire passcode entry to connect to discovered devices

[Copilot]

The TUI discovered nodes via mDNS but Enter only printed a placeholder — make_connection was never called. This adds an interactive passcode entry flow so users can pair with a discovered device.

Changes

• AppMode enum (Normal | EnteringPasscode { input: String }) added to App to track input state
• Passcode entry mode: pressing Enter on a selected node switches to EnteringPasscode; the output pane renders a live input field with cursor glyph
• make_connection wired up: on confirmation, calls node.make_connection(&discovered_node, &passcode) and displays ✓/✗ result
• Esc to cancel, status bar updates dynamically to reflect current mode

Node:    my-device
Address: 192.168.1.42
Port:    52341

Enter pairing code (shown on the target device):
  > 1234█

[Enter] confirm   [Esc] cancel

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