feat: New export backup UI - WPB-15211

[KaterinaWire]

Issue

In preparation for cross-platform backup, we need to update the UI for exporting a backup. This PR contains 2 major changes.

1. Password for backup export is now Optional.
2. The UI for exporting backup:
   Old:

New:

Design

Note: the following PR will contain a backup restore from the Settings logic.

Additional improvements:

1. Refactoring backupActiveAccount() method from the SessionManager and all the connected methods. I simplified the code by removing completion handlers and aligns with throwable and synchronous method design patterns.
2. All the new logic is in the WireUI -> WireSettingsUI -> Account -> Backup
3. Remove old code.

Testing

Settings -> Account -> Backup or Restore -> Backup now

Checklist

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

---

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.

[github-actions]

Test Results

    4 files     13 suites   8m 48s ⏱️
5 619 tests 5 590 ✅ 29 💤 0 ❌
5 630 runs  5 601 ✅ 29 💤 0 ❌

Results for commit 1e134af.

♻️ This comment has been updated with latest results.

[El-Fitz]

Well done! I've left a few comments, but nothing blocking.