feat: add multiple file browser selection

[maartenbreddels]

Summary

• Refactor FileBrowser around a reusable internal base while preserving single-selection behavior and string-directory compatibility.
• Add FileBrowserMultiple for Path-only multiple selection with optimistic row highlighting and selected-row ripple handling.
• Update the website docs demo/API docs and add focused tests, including a generic fake filesystem source.

Usage snippets

Single file selection keeps the existing FileBrowser API:

from pathlib import Path
from typing import Optional, cast

import solara

selected_file = solara.reactive(cast(Optional[Path], None))
solara.FileBrowser(Path.cwd(), selected=selected_file)

Multiple file selection uses FileBrowserMultiple and Path values:

from pathlib import Path
from typing import List, cast

import solara

selected_files = solara.reactive(cast(List[Path], []))
solara.FileBrowserMultiple(Path.cwd(), selected=selected_files)

Callback style also receives Path values:

from pathlib import Path
from typing import List

import solara


def on_select(paths: List[Path]):
    print(paths)


solara.FileBrowserMultiple(Path.cwd(), on_paths_select=on_select)

Demo

• Video demo: selecting report.csv and notes.txt, toggling report.csv, and clearing the remaining selection.

filebrowser-multiple-pr-demo.mp4

Tests

• uv run pytest tests/unit/file_browser_test.py::test_file_browser_callback_can_select tests/unit/file_browser_test.py::test_file_browser_relative_path tests/unit/file_browser_test.py::test_file_browser_programmatic_select tests/unit/file_browser_test.py::test_file_browser_multiple_callback_select tests/unit/file_browser_test.py::test_file_browser_multiple_programmatic_select tests/unit/file_browser_test.py::test_file_browser_multiple_open_and_navigate_clear_selection tests/unit/file_browser_test.py::test_file_browser_multiple_does_not_select_parent tests/unit/file_browser_test.py::test_file_browser_multiple_requires_path_directory tests/unit/file_browser_test.py::test_file_browser_base_supports_generic_source -q
• Commit hooks: passed on the squashed commit