fix(tui): ignore letter shortcuts when ctrl or alt is held - #669

Problem

Letter shortcuts compared the typed character without checking modifiers, so a ctrl or alt combo ran the plain-letter action:

• ctrl+r toggled raw value in the line editor and reset styling in Edit Colors.
• ctrl+d deleted a widget, and ctrl+a opened the add-widget picker. That last one matters for anyone whose tmux prefix is ctrl+a: pressing ctrl+a twice sends a literal ctrl+a through.
• On macOS, alt/option+←/→ is commonly sent as ESC b / ESC f, which Ink reads as alt+b / alt+f. Word-jumping with option-arrow could therefore toggle bold, background mode, and so on.
• A few handlers had already patched ctrl+s one key at a time with && !key.ctrl. Widget keybinds checked ctrl but not alt.

Fix

getPlainInput(input, key) in utils/input-guards.ts returns the typed character only for a bare keypress, and '' while ctrl or alt/option is held. Every letter shortcut now compares against it:

• the line editor's built-in shortcuts and the widget-specific keybind matching
• Edit Colors, Global Overrides, and Powerline setup, themes and separators
• the line selector and the hide-state checklist

These are unchanged:

• arrows, Enter and ESC, so option+arrow still navigates
• shift, so A/a handlers keep working
• text entry, which was already guarded by shouldInsertInput
• App's real combos, ctrl+c and ctrl+s

The ad-hoc !key.ctrl patches are gone, and modifier combos are free for bindings of their own.

Modifiers on macOS and Windows/Linux

Ink 6.2 reports ctrl, meta (alt/option) and shift:

• Guarding ctrl and meta covers Ctrl, PC Alt, and Mac Option when "Option as Meta/Esc+" is on.
• Mac Option in its default mode types a different character (∫, ®, …), which never matches a shortcut letter.
• Cmd and the Windows key normally aren't passed to terminal programs.
• AltGr arrives as the resulting character, so it's unaffected.

Testing

• getPlainInput unit tests.
• Line-editor handler tests: ctrl and alt held with each of a i d k c r m do nothing, the bare letter still works, and widget keybinds ignore alt as well as ctrl.
• hotkey-modifiers.test.ts: a table-driven Ink test that sends the real terminal bytes. Ctrl is the control character, e.g. \x12 for ctrl+r; alt is ESC + key. For one shortcut in each of the seven components, it checks that the bare key fires and ctrl and alt don't. ctrl+i isn't used because it's the same byte as Tab.
• On today's main (with docs: explain an empty status line in one folder (workspace trust) #606 and feat(git-is-fork): render forks as an editable glyph #617 merged in locally): bun test gives 2809 pass, 0 fail (Bun 1.4.2), and bun run lint is clean. Under Node 26 vitest, the 3 touched test files pass (70).
• Built CLI under Bun 1.4.2 and Node 26.10.0: all runtimes agree with each other and with main in plain and Powerline modes, and the TUI opens and exits under both.
• Manual run in tmux, which sends genuine M- and C- sequences: alt+b, ctrl+r and alt+d do nothing in the line editor, while plain b and r work.