fix(mobile): support pre-Liquid-Glass iOS bottom toolbar

[gabrielelpidio]

What Changed

Added a fallback bottom toolbar for iOS versions before Liquid Glass support. Older iOS versions now use standard navigation search and toolbar primitives, while iOS 26 continues using the native Liquid Glass toolbar integration.

Why

The native mail-style toolbar is only supported by the iOS 26 Liquid Glass implementation. On older iOS versions, those toolbar items were silently dropped, leaving search and thread actions unavailable. This restores the expected controls across supported iOS versions.

UI Changes

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

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Note

Low Risk
UI-only navigation chrome and list inset changes on older iOS; Liquid Glass paths stay gated behind the existing capability flag with no auth or data-layer impact.

Overview
On iOS without Liquid Glass, the native mail-style header toolbar was dropped entirely, so search, filters, and new-task were missing. This PR gates that integration behind NATIVE_MAIL_SEARCH_TOOLBAR_SUPPORTED and restores standard pull-down search plus a bottom NativeHeaderToolbar on home (and aligned behavior on archived threads and thread files).

Home moves filter/sort and new-task into the fallback bottom bar; thread list v2 still hides sort menus when sorting is ignored. HomeScreen adds 44pt list padding and scroll inset clearance so list content isn’t covered by the overlay toolbar when automatic inset adjustment is off.

StackHeader supports a flexible spacer and assigns spacing item index so spacers don’t jump ahead of earlier toolbar siblings.

^{Reviewed by Cursor Bugbot for commit 591a7f1. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Fix mobile home screen layout and toolbar for pre-Liquid-Glass iOS

• Adds NATIVE_MAIL_SEARCH_TOOLBAR_SUPPORTED flag (aliasing NATIVE_LIQUID_GLASS_SUPPORTED) to gate use of the integrated mail-style toolbar to iOS 26+.
• On pre-Liquid-Glass iOS, HomeHeader falls back to a pull-down search bar and renders a bottom toolbar with filter menus and a compose button instead of the integrated toolbar.
• HomeScreen reserves 44pt of bottom padding on pre-Liquid-Glass iOS to prevent list content from being obscured by the bottom toolbar overlay.
• ArchivedThreadsScreen and ThreadFilesRouteScreen similarly gate compact mail toolbar and integratedButton search placement behind the new flag.
• Fixes NativeHeaderToolbarSpacer to support a flexible prop and corrects insertion-order bugs for spacing items in native header toolbars.

^{Macroscope summarized 591a7f1.}

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR changes runtime UI behavior across multiple screens to support pre-Liquid-Glass iOS versions, including conditional toolbar rendering, search placement changes, and padding adjustments. The multi-screen scope and feature flag interactions warrant verification that both old and new iOS versions behave as expected.

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[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 1 potential issue.

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