feat(mouse): triple-click selects the whole line

[roramirez]

Summary

Adds triple-click line selection. Today double-click selects a word; a third rapid click now selects the entire line under the cursor, leaves it highlighted in anchored Visual mode, and copies it to the clipboard — mirroring the existing double-click copy behavior.

Changes

• src/app_state.rs: last_click now carries a running click count (Option<(Instant, f64, f64, u8)>). New AppState::click_count returns 1/2/3 within the 400 ms / 4 px window (a 4th rapid click cycles back to a fresh single; a press outside the window resets to 1).
• src/app_event.rs: replaced is_double_click with a click_count-driven dispatch in handle_selection_click — 3 → line select, 2 → word select, 1 → start drag selection.
• src/input/mouse_ops.rs: new select_line_at that anchors a full-row Visual selection (start_col = 0, cur_col = cols-1) and copies it.
• CHANGELOG.md: entry under [Unreleased] → Added.
• Tests: click-count helper (single/double/triple/wrap/timing/distance) and select_line_at producing a full-row anchored Visual selection.

How to test

• cargo test — new tests: click_count_* in src/app_state_test.rs and select_line_at_* in src/input/mouse_ops_test.rs.
• Manual: triple-click a line of terminal output — the whole line highlights and is copied to the clipboard; double-click still selects a word; single-click-drag still selects a range.