Improve launch sequence for custom apps

[BPerlakiH]

Fixes: #1006

There must be quite some changes to achieve this.
Custom apps (the ones without catalog / library capability) have a different loading
sequence compared to Kiwix app, since we do not have the "Fetching catalog" part.

Also the views were too tightly coupled with the loading states and library. Eg: Welcome screen was coupled with WebView.

I separated out the sequence of loading and its possible states into a new place, and wrapped that into a protocol with 1 implementation for Kiwix, and one for custom apps without Library.

I have re-tested both Kiwix and custom apps on iPhone iPad and macOS.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 43.52518% with 157 lines in your changes missing coverage. Please review.

Project coverage is 38.26%. Comparing base (4cffee9) to head (ba3aef1).

Files with missing lines	Patch %	Lines
Views/WelcomeCatalog.swift	0.00%	63 Missing ⚠️
ViewModel/LaunchViewModel.swift	55.29%	38 Missing ⚠️
App/CompactViewController.swift	48.78%	21 Missing ⚠️
Views/BrowserTab.swift	55.31%	21 Missing ⚠️
Views/LocalLibraryList.swift	59.25%	11 Missing ⚠️
ViewModel/BrowserViewModel.swift	62.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1008      +/-   ##
==========================================
- Coverage   39.86%   38.26%   -1.60%     
==========================================
  Files         116      118       +2     
  Lines        6618     6879     +261     
==========================================
- Hits         2638     2632       -6     
- Misses       3980     4247     +267     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.