feat: add configurable keyboard shortcuts (#211)

[matheuswhite]

What

Closes #211. Adds an optional [shortcuts] table to config.toml that lets users remap the action/navigation keyboard shortcuts, merged over the built-in per-platform defaults (an omitted action keeps its default).

[shortcuts]
record        = "Ctrl+G"   # move record off Ctrl+R
next_bookmark = "F2"
prev_bookmark = "F3"

15 remappable actions: copy, clear, save, record, search_toggle, toggle_case, toggle_regex, page_up, page_down, jump_start, jump_end, word_left, word_right, next_bookmark, prev_bookmark. Text-editing/control keys (typing, Enter, Esc, arrows, Home/End, Backspace/Delete, headless Ctrl+K/Ctrl+Q) stay fixed.

How

• src/inputs/keymap.rs (new) — Action / KeyBinding / Keymap own all key knowledge; config.rs stays a plain BTreeMap<String,String> and never enumerates the action set.
• Logical modifiers — config combos ("Ctrl+Left", "Shift+Tab", "F2") are parsed once into the physical crossterm event of the build target, so a config file is portable: the macOS Ctrl+arrow → Alt+arrow terminal quirk is handled in resolve, and the Windows jump_* defaults are Ctrl+PageUp/Down (vs Alt elsewhere). Matching is exact on modifiers, case-insensitive on chars. A default_matches_current_arms test proves the default keymap reproduces the old hardcoded arms per platform.
• handle_key_input resolves a remappable action before the intrinsic key match; a Tab-bound next_bookmark falls through to @tag autocomplete while the pop-up is up. The old CTRL_MODIFIER/ACTION_MODIFIER consts are gone (only ACTION_MODIFIER, for the fixed Alt+Enter arm, remains).
• Validation — unknown action names, unparseable combos, reserved-key overrides and duplicate bindings are fatal config errors routed through main's existing [ERR] funnel.

Behavior change

An unbound Ctrl/Alt+letter is now swallowed rather than typed literally (e.g. Ctrl+C after copy is remapped away). AltGr (Ctrl+Alt together, the Windows international-layout signature) still types.

Note: terminals send Ctrl+I/Ctrl+J/Ctrl+M as Tab/Enter, so those combos can't be used as shortcuts (documented).

Tests

• keymap.rs unit tests: parser round-trips & rejections, per-platform resolve, exact/case-fold matcher, merge/swap, unknown/reserved/duplicate errors, and the anti-regression default-parity table.
• config.rs: [shortcuts] parsing.
• inputs_task.rs: the char_is_literal_text typing/AltGr predicate.
• tests/tui_e2e.rs: a config-driven remap (record → Ctrl+G, proving override + old-key disabled) and a headless Ctrl+F-swallow test. The e2e harness now isolates the config dir to a temp tree.

All 226 unit + 14 e2e tests pass (1 e2e ignored on macOS per the existing PTY-baud limitation); tree is warning-clean under #![deny(warnings)]. Docs updated (README shortcut table gains a "Config action" column + a "Custom shortcuts" section; CLAUDE.md).

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