feat: character-level chat selection that persists after mouse release

[wowi42]

Summary

Implements item 4 of the "Input and pickers" section of #186 ("Support character-level selection: mouse drag and y copy currently select whole visual rows only") and fixes a UX bug where the selection disappeared the moment the mouse button was released.

Changes

Character-level selection

• Selection endpoints are now (visual_line, display_col) points instead of whole-row LineEntry indices; MouseDrag honors the column it previously ignored.
• The viewport renders the selection as a partial-line reversed segment: first/last lines sliced to their columns, middle lines fully covered. Slicing is display-column based and CJK-aware (wide chars straddling a boundary are excluded), via new byte_index_at_display_col / display_col_slice helpers in ui::utils.
• selected_text() (and therefore y copy) extracts exactly the highlighted range, normalizing reversed/upward drags.

Selection persists after mouse release, and is copied on release

• Previously MouseUp copied to clipboard and immediately cleared the selection, so the highlight vanished on release. Now the drag ends (selection_dragging tracks the button state separately from selection_active) but the highlight stays: y copies again, Esc clears, the next click starts a new selection.
• The selection is also copied to the system clipboard on release via copy_to_clipboard (pbcopy/wl-copy/xclip/clip.exe): while mouse capture is enabled the terminal emulator has no native selection, so Cmd+C (macOS) / Ctrl+Shift+C (Linux) has nothing to grab — the app must write the pasteboard itself.

Verification

• cargo test (720 passed), cargo test --no-default-features (605), cargo clippy --all-features and --no-default-features with -D warnings — all clean.
• New unit tests: display-column helpers (incl. CJK), char-level selected_text (first/last line slicing, reversed drag, empty point selection).
• Smoke-tested on a pty with synthetic SGR mouse events: partial-line reverse highlight renders, selection survives release, y copies it.

Refs #186

[wowi42]

Follow-up (0fa1d4d): selection is now also copied to the system clipboard on mouse release — with mouse capture enabled the terminal emulator has no native selection, so Cmd+C/Ctrl+Shift+C has nothing to copy and the app must write the pasteboard itself. The highlight still persists for y/Esc. Verified on macOS: pbpaste contains the selected text after release.