feat(files): support Ctrl+A select-all in the file list

[ousugo]

What this PR does

Before this PR:

• The Files page offers select-all only through the header checkbox; there is no keyboard shortcut for it.

After this PR:

• Pressing Ctrl+A (Cmd+A on macOS) in the Files page list view selects every file in the current filtered view, with the same effect as checking the header select-all checkbox.
• The shortcut is guarded the same way as the page's existing Delete/F2 shortcuts: it does nothing while an input, text field, or other interactive control has focus (native text select-all is preserved), and it stays inert in the image grid view, which has no selection model.

Related to #9616 (file-management batch operations, closed as completed when the batch-selection UI shipped — this PR completes it with a keyboard select-all entry point).

Why we need it and why it was done in this way

The following tradeoffs were made:

• Reused the existing select-all logic behind the header checkbox (handleSelectAllVisible) instead of adding a new selection path, so the checkbox and the keyboard shortcut share one owner for select-all semantics.
• The shortcut always selects (never toggles), matching standard file-manager behavior; deselection remains available through the header checkbox.

The following alternatives were considered:

• Handling the shortcut in the list component: rejected because selection state and the page's existing keyboard shortcuts already live on the page, making the page-level keydown handler the single owner.
• Adding the shortcut to the image grid view: rejected for now because that view has no selection model; the branch explicitly keeps the shortcut inert there to avoid invisible selections.

Links to places where the discussion took place: N/A

Breaking changes

None.

Special notes for your reviewer

• Regression tests cover Cmd+A on macOS, Ctrl+A outside macOS, the interactive-control guard, and the image-grid no-op path, written with userEvent.keyboard and role-based queries per docs/references/testing/frontend-testing.md.
• The image-grid no-op test is mutation-verified: removing the !isImageGrid clause makes it fail (file.batch_trash gets called), so the guard clause is genuinely pinned.

Checklist

This checklist is not enforcing, but it's a reminder of items that could be relevant to every PR.
Approvers are expected to review this list.

• Branch: This PR targets main
• PR: The PR description is expressive enough and will help future contributors
• Code: Write code that humans can understand and Keep it simple
• Refactor: You have left the code cleaner than you found it (Boy Scout Rule)
• Upgrade: Impact of this change on upgrade flows was considered and addressed if required
• Documentation: A user-guide update was considered and is present (link) or not required. Check this only when the PR introduces or changes a user-facing feature or behavior.
• Self-review: I have reviewed my own code (e.g., via /gh-pr-review, gh pr diff, or GitHub UI) before requesting review from others

Release note

Files: You can now press Ctrl+A (Cmd+A on macOS) in the file list to select all files in the current view.