fix(panel): position cursor on active file when toggling panel open

[dlyongemallo]

Fixes #161.

[Copilot]

Pull request overview

Fixes the inconsistency described in #161 where opening the file panel via :DiffviewToggleFiles (when file_panel.show = false) would not position the cursor on the currently active file entry.

Changes:

• Update FilePanel:open() to re-highlight (and thus reposition the cursor on) the current active file when the panel transitions from closed → open.
• Add a functional regression test ensuring toggling the panel open places the cursor on the active file entry.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
lua/diffview/scene/views/diff/file_panel.lua	Ensures open() positions the cursor on the active file by calling highlight_cur_file().
lua/diffview/tests/functional/cdiff_view_spec.lua	Adds a regression test for cursor positioning when the panel is toggled open with file_panel.show = false.

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