feat: make Shift+Enter insert a newline by enabling terminal key modes

[wowi42]

Problem

Shift+Enter arrived as plain Enter in terminals that require keyboard mode enablement, so it submitted the prompt instead of inserting a newline. Enabling those modes also changes other keys: the original PR made Kitty Ctrl-C insert [99;5u into the composer, and xterm level 1 did not report Shift+Enter distinctly.

Fix

• Enable Kitty keyboard disambiguation and xterm modifyOtherKeys level 2. Query xterm's prior mode and restore its reported value on exit; fall back to its configured default if it does not answer. Skip non-TTY output, TERM=dumb, and Windows.
• Map the modified Ctrl, Alt, Escape, and Shift keys into prompt_toolkit, including Unicode text, so existing shortcuts and ordinary input keep working.
• Suspend lecode's keyboard modes while Ctrl-G opens an external editor. Keep draft and session cleanup guaranteed if terminal restoration fails.

Validation

• Ruff check and format checks passed.
• Focused TUI tests cover Shift+Enter, Ctrl-C, xterm Unicode Shift/Alt input, mode restoration, editor handoff, and cleanup.
• tests/test_tui_streaming_pty.py: 16 passed.
• The full test_tui_app.py run is not green: generated-prompt hook cases fail, and test_pipe_ctrl_c_exits_cleanly passes but hangs after the test. The latter hang also occurs on an untouched main snapshot.

[Mouhand-Kaddo]

Confirmed by testing the installed PR commit 90506f0965f761e13676451b9e706d241f3e7742: enabling Kitty keyboard disambiguation breaks existing shortcuts.

Reproduction on a terminal supporting the Kitty protocol:

1. Start lecode and type some text.
2. Press Ctrl-C.
3. [99;5u is inserted into the prompt instead of clearing it.

_ENABLE_KEY_MODES enables flag 1 (CSI > 1 u), which changes Ctrl/Alt/Escape key encodings. _register_shift_enter() only adds support for Shift-Enter. The pinned prompt_toolkit parser treats the other CSI-u sequences as Escape followed by literal text. Ctrl-D, Ctrl-J, Escape, and Alt-Enter are also affected in parser reproductions.

Please support the CSI-u encodings of the existing controls before enabling this mode, and add regression coverage for Ctrl-C cancellation/clearing and the other affected shortcuts. The current tests supply legacy shortcut bytes, so they do not catch this regression.

Relevant code:

lecode/src/lecode/tui/app.py

Line 201 in 90506f0

_ENABLE_KEY_MODES = "\x1b[>1u\x1b[>4;1m"

Protocol reference: https://sw.kovidgoyal.net/kitty/keyboard-protocol/#disambiguate-escape-codes