fix: keep File Explorer highlight in sync with the active file

[suhailopensource]

Problem

Closes #528. In the File Explorer a row's highlight comes from two independent sources — the active document (activeFile.source) and react-arborist's own selection (node.isSelected, set on click and never cleared). Clicking a file sets both, but every other way of changing the active document leaves the selection stale:

• Close all tabs → activeFile becomes null but the last-clicked row stays selected, so it remains highlighted.
• Switch tabs from the tab bar → activeFile moves while the tree selection stays on the last-clicked file, so two rows are highlighted at once.

Fix

Drive the tree selection from the active document in the existing activeFile?.source effect (apps/ui/src/core/file-system/components/FileSystemList.tsx):

• No active document → deselectAll(), so no row stays highlighted.
• Active document → select(source, { focus: false }) (replacing the bare scrollTo), which clears any previous selection and scrolls the row into view, so exactly one row — the active file — is lit. focus: false keeps it a purely visual sync, and a cancelled guard avoids a stale select during rapid tab switches.

Testing

• Verified manually in the running app: closing all tabs clears the highlight, and switching tabs never leaves two rows highlighted.
• yarn typecheck (apps/ui): no new type errors from this change.

[phurpa-tsering]

hey @suhailopensource ,

Two issues with Shift+click range selection here:

1. Breaks the selection anchor — FileSystemList.tsx:615: opening a file sets react-arborist's selection anchor, then this effect's deselectAll() fires on the resulting activeFile.source change and wipes it. Since selectContiguous() needs that anchor, Shift+click right after opening any file selects nothing.

2. Highlighting is inconsistent even when selection works — TreeNode.tsx:459: the active/open file always gets the single-selection bg-active style instead of the multi-select bg-accent/20 style, so a file range never looks fully highlighted the way a folder range does.

Worth a manual pass on file Shift+click before merging — these compound.

[suhailopensource]

hey @suhailopensource ,

Two issues with Shift+click range selection here:

1. Breaks the selection anchor — FileSystemList.tsx:615: opening a file sets react-arborist's selection anchor, then this effect's deselectAll() fires on the resulting activeFile.source change and wipes it. Since selectContiguous() needs that anchor, Shift+click right after opening any file selects nothing.
2. Highlighting is inconsistent even when selection works — TreeNode.tsx:459: the active/open file always gets the single-selection bg-active style instead of the multi-select bg-accent/20 style, so a file range never looks fully highlighted the way a folder range does.

Worth a manual pass on file Shift+click before merging — these compound.

working on it will address it by today.

[suhailopensource]

@phurpa-tsering done addressed ur review comments