fix: Open active file in external app instead of repo directory

[charlesvien]

Problem

Clicking "Open in [app]" always opened the repo root directory, even when a specific file was active in the editor.

Changes

1. Derive active file path from panel layout store in TaskDetail
2. Join relative path with repo path as the external app target, fall back to repo directory
3. Add fixed overlay to prevent drag-through when dropdown menu is open
4. Fix dead diff type check and normalize path joining

How did you test this?

Manually

[charlesvien]

• fix: Show parent directory in file mention chips #1472
• fix: Fix Enter key blocked after dismissing empty suggestion popup #1471
• fix: Open active file in external app instead of repo directory #1470 👈 (View in Graphite)
• fix: Rename bypass permissions to auto-accept permissions #1469
• fix: Polish message editor, settings dialog and minor UX tweaks #1468
• main

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[charlesvien]

Merge activity

• Apr 3, 1:05 AM UTC: A user started a stack merge that includes this pull request via Graphite.
• Apr 3, 1:13 AM UTC: Graphite rebased this pull request as part of a merge.
• Apr 3, 1:18 AM UTC: @charlesvien merged this pull request with Graphite.