feat: support no default backend app - WPB-26207

[github-actions]

This PR was automatically cherry-picked based on the following PR:

• feat: support no default backend app - WPB-26207 #4974

Original PR description:

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WPB-26207 [iOS] Landing screen for Gov edition

Issue

The new Gov edition does not contain any email.json or url.json nor default backend embedded.

Changes:

• Make Backend.bundle optional at runtime and introduce fallback BackendEnvironment / URL / email behaviors when it’s absent.
• Add new authentication steps + UI to configure a backend via configuration link or QR code, plus a post-configuration “Continue” screen.

Testing

1. ./setup.sh -o './wire-ios-build-assets/Gov/AppStore'
2. enter/scan deeplink

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Checklist

• Title contains a reference JIRA issue number like [WPB-XXX].
• Description is filled and free of optional paragraphs.
• Adds/updates automated tests.

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UI accessibility checklist

If your PR includes UI changes, please utilize this checklist:

• Make sure you use the API for UI elements that support large fonts.
• All colors are taken from WireDesign.ColorTheme or constructed using WireDesign.BaseColorPalette.
• New UI elements have Accessibility strings for VoiceOver.

[zenkins]

❌ Cannot auto-approve: conflict markers (e.g. <<<<<<<, >>>>>>>) were found in the changes. Please resolve them before merging.

[netbe]

This PR needs to be redone as for 4.X the screens changed

[github-actions]

Test Results

0 tests   0 ✅  0s ⏱️
0 suites  0 💤
0 files    0 ❌

Results for commit 4e9564d.

Summary: workflow run #29496743115
Allure report (download zip): html-report-32047-feat_wire-gov-cherry-pick