Issue for this PR

Closes #29875
Closes #29951

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Restores the desktop session header's Open in control in both titlebar layouts.

Two regressions overlapped:

1. The V2 titlebar introduced in feat(app): refine desktop v2 home and session controls #28788 rendered only status and review actions, leaving the existing Open-in control in the legacy branch.
2. feat(app): allow toggling tabs layout #29526 changed isDesktopV2 from a boolean to a Solid memo, but the <Show> condition still passed the accessor itself. Since the function reference is always truthy, disabling the new layout could not reach the legacy fallback.

This PR:

• calls isDesktopV2() so the legacy fallback works again;
• renders the existing open-app workflow beside SessionHeaderV2Actions using V2 menu/button primitives;
• keeps the menu in SessionHeader, where its app detection, persisted selection, request state, and callbacks already live, instead of expanding SessionHeaderV2ActionsState with pass-through fields;
• adds the V2 copy icon used by the Copy path menu item.

The restored menu supports the native file manager, detected editors/terminals, and Copy path. The underlying openPath integration is unchanged.

Acknowledgements:

• fix(app): add open folder button to V2 desktop session header #29888 by @ComradeSwarog identified and implemented the missing V2 Open-in control. This PR carries that fix forward onto current dev with a smaller state boundary.
• fix(session-header): call isDesktopV2() memo in Show when predicate #31875 by @EdwardYuen identified the Solid memo accessor bug that made the legacy fallback unreachable. This PR includes the same one-line correction.

Related reports: #29829, #29867, #31101, #31214, #31458, #31507, and #31878.

How did you verify your code works?

• Ran the Electron desktop app against this repository.
• Reproduced the original state: V2 showed only the review action and had no Open options control.
• Verified V2 now shows the selected app action and dropdown.
• Disabled New layout and designs and verified the legacy Open-in, terminal, status, and file-tree controls render again.
• Opened the dropdown and verified Finder, detected applications, and Copy path.
• Invoked Open in Finder successfully with no error toast.
• Ran the full packages/app unit suite: 376 tests passed across 66 files.
• Ran the full packages/ui suite: 27 tests passed across 6 files.
• Ran bun typecheck in packages/app and packages/ui.
• Ran the app production build.
• Passed the repository pre-push workspace typecheck: 23 tasks passed.

Screenshots / recordings

Before, the V2 session header had no Open-in control:

After, the V2 Open-in menu is restored:

Both screenshots were captured from the local Electron verification for this PR.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR