feat(mouse): Shift+Click extends the selection

[roramirez]

Summary

A plain left-press starts a fresh selection. This adds Shift+Click: when Shift is held at press time and an existing Visual selection is present, the selection is extended instead of reset — the original anchor (start_col/start_row) is kept and the cursor (cur_col/cur_row) jumps to the clicked cell, so the selection grows from the anchor to the click point. Dragging afterward continues to extend, and release copies. With no prior selection, it falls back to a normal fresh selection from the click.

Changes

• src/input/mouse_ops.rs: add extend_mouse_selection (resolves the clicked pane + cell, extends the current Visual selection, sets mouse_selecting = true) and a pure, testable extend_visual_mode helper that computes the new mode.
• src/app_event.rs: in handle_selection_click's Pressed branch, read Shift via self.modifiers.state().shift_key(); if held, call extend_mouse_selection, otherwise keep the existing double/single-click dispatch.
• src/input/mouse_ops_test.rs: new tests covering extend-from-anchored-Visual (anchor preserved, cursor moved), extend-from-unanchored-Visual, and no-selection returning None for a fresh start.
• CHANGELOG.md: add entry under [Unreleased] / Added.

How to test

• cargo test — all pass, including the three new mouse_ops_test cases.
• Manually: make a Visual selection with the mouse (or v in Normal mode), then Shift+Click elsewhere in the pane — the selection extends from the original anchor to the click point rather than resetting; dragging continues to extend and releasing copies. A plain click (no Shift) still starts a fresh selection, and double-click still selects a word.