feat(tui): keyboard text selection + copy in transcript

[alecuba16]

Summary

Split B of #387. Adds keyboard-driven text selection in the transcript pane.

This PR is the second half of the #387 split. Split A is #407 (session browser toggles + keyboard selection). No shared payload between the two.

How it works

1. Press v on an empty prompt to enter selection mode. A cursor bar appears at the bottom of the transcript.
2. Use ↑/↓ to extend the selection. PageUp/PageDown scroll by a page, Home/End jump to the top/bottom.
3. Enter or Ctrl+C copies the selected text to the clipboard and exits selection mode.
4. Esc cancels without copying.

Review feedback addressed

Every item from the #387 review is fixed:

#	Review issue	Fix
3	Bare v on empty prompt eats first char of messages starting with v, shadows vim's v	v handler is gated on voice_recorder.is_none() and !matches!(vim_mode, Normal/Visual/VisualBlock), mirroring the ? help toggle pattern
4	KeyContext::Transcript bypassed — keys were hardcoded KeyCode matches	All selection keys route through KeyContext::Transcript in keybindings.rs. current_key_context() returns Transcript when kb_select_mode is active
5	Up/Down scroll branches unreachable	Replaced with move_kb_cursor(delta) that clamps to last_selectable_area bounds and scrolls to keep the cursor visible
6	Docs described non-existent features (hjkl, Shift-extend, Enter-yank)	Docs now accurately describe v entry, arrow/PageUp/Down/Home/End navigation, Enter/Ctrl+C copy, Esc cancel
7	Session browser half never runs (dead App::run code)	Fixed in #407 (split A)
8	Home scrolls to bottom because total_message_lines is always 0	total_message_lines is now written at render time in render_messages
9	last_selectable_area is the whole terminal, not the transcript	render_app now sets last_selectable_area to last_msg_area (falls back to full frame on welcome screen)
10	No visible cursor on entry	apply_selection_highlight shows a cursor bar when anchor == focus instead of early-returning
11	_ => return false swallows Ctrl+D	Guard at the top of the hardcoded section allows Ctrl+D through; all other unbound keys are swallowed
12	Preview panel gets clipped	Fixed in #407 (split A)
13	Test coverage never drives real key events	13 new tests driving handle_key_event with real KeyEvents

Files changed

• crates/core/src/keybindings.rs — Transcript context bindings updated: selectionUp/Down/PageUp/PageDown/GoStart/GoEnd/Copy/Cancel. 2 new core resolver tests.
• crates/tui/src/app/keys.rs — v entry handler, selection mode guard, current_key_context returns Transcript, action handlers, helper methods (enter_kb_selection, exit_kb_selection, move_kb_cursor, update_kb_selection)
• crates/tui/src/app/mod.rs — kb_select_mode and kb_cursor_row fields
• crates/tui/src/render.rs — last_selectable_area restricted to last_msg_area, total_message_lines written at render time, apply_selection_highlight shows cursor bar when anchor == focus
• crates/tui/src/app/tests.rs — 11 new TUI tests (8 synthetic + 3 render pipeline driving real TestBackend/render_app/handle_key_event)
• docs/commands.md — keyboard selection section

Tests

13 new tests total:

• 2 core keybinding resolver tests (crates/core/src/keybindings.rs): test_transcript_context_resolves_selection_actions drives the real KeybindingResolver::new(&UserKeybindings::default()) and asserts all 8 Transcript actions (selectionUp/Down/PageUp/PageDown/GoStart/GoEnd/Copy/Cancel) resolve correctly. test_transcript_context_does_not_resolve_chat_actions verifies context isolation (up in Chat resolves to historyPrev, not selectionUp).
• 8 TUI synthetic unit tests: enter with v, keys swallowed in selection mode, cancel with Esc, context routing, Ctrl+D not swallowed, cursor movement, clamping at bounds, copy exits mode.
• 3 TUI render pipeline tests using real TestBackend + Terminal + render_app: test_render_restricts_selectable_area_to_transcript asserts last_selectable_area.height < terminal.height, matches last_msg_area, and total_message_lines > 0 (was always 0 before). test_render_selectable_area_falls_back_on_empty_transcript verifies welcome screen fallback. test_kb_selection_full_pipeline_enter_move_copy renders messages, then drives v→up→down→enter through handle_key_event exercising the real keybinding resolver.

All 13 pass. Clippy clean. Cargo check clean.

• Pre-existing failure settings_screen::tests::all_entries_returns_expected_settings (expects ≤20 settings, gets 21) is unrelated — fails on main, this PR does not touch settings.

CRLF preservation

keybindings.rs and docs/commands.md are CRLF files. All additions use \r\n. No LF churn on untouched lines.