Polish file browser appearance and add screenshot tests

[matchaboar]

Summary

Focused UI appearance and screenshot-testing split from #43, rebased onto current main.

• Add the scoped folder header and refresh affordance.
• Add explicit loading, empty-folder, and error/retry states.
• Strengthen active-file treatment and expose the related File Browser settings.
• Restore standard macOS title-bar double-click behavior.
• Add light, dark, empty, and error screenshot regression baselines.
• Keep performance architecture, Git metadata collection, SSH profiles, and asset fetching out of this PR.

CI permissions

Tests now run in a separate tests job with permissions: contents: read and actions/checkout configured with persist-credentials: false. The release/build job remains separate and no longer executes tests. Its checkout also disables persisted credentials; the appcast push receives an explicit token only in the tag-only step that needs it.

Red/green and screenshot runs

Environment: macOS 26.6.1 arm64, Swift 6.3.1.

Run	Command	Result	Duration
Red	./run-screenshot-tests.sh	Failed: all four baselines missing	2.11s
Record	./run-screenshot-tests.sh --record	Passed; baselines recorded	1.66s
Verify 1	./run-screenshot-tests.sh	Passed	1.63s
Verify 2	./run-screenshot-tests.sh	Passed	1.77s
Verify 3	./run-screenshot-tests.sh	Passed	1.68s
UI logic red	./run-tests.sh	Failed before presentation/settings/title-bar implementations	0.39s
UI logic green	./run-tests.sh	Passed 14 focused UI checks plus existing harness	3.21s
App build	./build.sh	Passed	16.88s

The populated light/dark baselines cover the folder header, refresh control, expanded hierarchy, and active-file treatment. Empty and error baselines cover the remaining sidebar states. These are screenshot/correctness results, not performance claims.

[nonatofabio]

This looks good overall. Before merging, please rebase onto current main and resolve the conflicts, keeping the v3.2 tab reordering and heading spine changes intact. This and #45 both introduce FileBrowserView.swift, so we also need to line up those changes so neither gets lost.

The Swift sources pass type-checking locally, but there are no CI results on this PR yet. Please get the build, logic tests, and screenshot checks passing on the rebased commit, and give the title-bar double-click behavior a quick manual check. Once that's done, I'm happy to take another look for merging.