Handle CR line endings in the query editor

[bolexyro]

Summary

• normalize CRLF and CR-only clipboard text to LF before inserting it into the query editor
• preserve newline widths when converting autocomplete cursor locations to string offsets
• support LF, CRLF, and CR-only text in both cursor-conversion directions
• add regression coverage for pasted line endings, cursor round trips, trailing line endings, and empty text

Root cause

The paste helper split clipboard content only on \n. CR-only clipboard content was therefore inserted unchanged even though Textual displayed it as multiple rows. That left the SQL highlighter and line-oriented editing operations with malformed input. Autocomplete also split only on \n, so Textual could report a multi-row cursor location while sqlit saw a single line, causing _location_to_offset to index past the line list.

User impact

Pasted multiline SQL now uses the editor's expected LF representation, so syntax highlighting remains correct. Autocomplete also no longer crashes if CR or CRLF text reaches its cursor conversion helpers, and the cursor remains correctly positioned after applying a completion.

Fixes #292.

Validation

• pytest tests/unit/test_autocomplete_cursor_positions.py tests/unit/test_query_paste.py --timeout=60: 10 passed
• pytest tests/unit -q --timeout=60: 1122 passed, 2 skipped
• ruff check on changed source and test files: passed
• mypy on changed source files: passed

Validation ran in a Python 3.12 container.