TUI: support multi-line paste via bracketed paste mode

[bug-ops]

Problem

Pasting multi-line text into the TUI input field does not preserve newlines. Crossterm bracketed paste mode is not enabled, so pasted text arrives as individual KeyCode::Char events with newlines stripped by the terminal.

Proposed solution

Enable crossterm bracketed paste and handle Event::Paste(text) to insert the full string at cursor position.

Changes

1. crates/zeph-tui/src/lib.rs — enable/disable bracketed paste:

   • Add crossterm::event::EnableBracketedPaste to the terminal setup sequence (alongside EnableMouseCapture)
   • Add crossterm::event::DisableBracketedPaste to the teardown/cleanup

2. crates/zeph-tui/src/event.rs — propagate paste events:

   • Add Paste(String) variant to AppEvent enum
   • Handle CrosstermEvent::Paste(text) => AppEvent::Paste(text) in the event reader

3. crates/zeph-tui/src/app.rs — insert pasted text:

   • Add AppEvent::Paste(text) arm in handle_event()
   • Insert the full string at byte_offset_of_char(cursor_position)
   • Advance cursor_position by text.chars().count()
   • This naturally preserves newlines in the pasted content (multi-line input already works via Shift+Enter)

Notes

• Bracketed paste is widely supported by modern terminals (iTerm2, Terminal.app, Alacritty, kitty, WezTerm)
• No config option needed — this should always be enabled
• Existing Shift+Enter newline insertion continues to work as before