feat(picker): support file line navigation

[fcoury-oai]

Why

File-picker queries currently treat a :<line> suffix as part of the filename, so pasting or typing a location such as sona:123 removes the intended file from the results and requires a separate navigation step.

What Changed

• Parse optional numeric line suffixes only in the file picker, while fuzzy matching, ranking, and highlighting continue to use the filename portion of the query.
• Open the selected file through the existing location-navigation action, converting one-based input to the editor's zero-based line position while preserving full query history and ordinary file opening.
• Keep results visible while an incomplete suffix is being typed, handle zero and oversized line numbers safely, and document the syntax in docs/GETTING_STARTED.md.
• Cover suffix parsing, incremental filtering, filename ranking, highlighting, query history, incomplete suffixes, and real-file navigation in src/ui/file_picker.rs.

How to Test

1. Open a workspace containing a source_name.rs file with at least 123 lines, press Ctrl-p, and type sona:123. Verify the file remains visible while typing :123; press Enter and confirm the cursor lands on line 123.
2. Reopen the picker, type sona:, and press Enter. Verify the file still opens normally without a line jump. Repeat with sona:999 for a shorter file and confirm navigation clamps to the final navigable line.
3. Run cargo test -p red ui::file_picker::tests --lib. The focused suite passes 25 tests; its existing manual performance benchmark remains ignored.