perf: use thumbnails on the home screen

[adil192]

Tasks

• Take screenshots
• Display screenshots and invert them if needed
• Fix the "_elements.contains(element)" Flutter assertion
• Don't display screenshots as separate files in recent notes/browse pages
• Fallback to old method if thumbnail file doesn't exist

[codecov]

Codecov Report

Attention: 88 lines in your changes are missing coverage. Please review.

Comparison is base (946602c) 36.10% compared to head (110e18f) 36.45%.

Files	Patch %	Lines
lib/components/home/preview_card.dart	0.00%	55 Missing ⚠️
lib/components/canvas/canvas_preview.dart	0.00%	14 Missing ⚠️
lib/pages/editor/editor.dart	76.08%	11 Missing ⚠️
lib/data/editor/page.dart	73.33%	4 Missing ⚠️
lib/components/settings/nextcloud_profile.dart	0.00%	2 Missing ⚠️
lib/data/file_manager/file_manager.dart	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1054      +/-   ##
==========================================
+ Coverage   36.10%   36.45%   +0.35%     
==========================================
  Files         107      107              
  Lines        8347     8360      +13     
==========================================
+ Hits         3014     3048      +34     
+ Misses       5333     5312      -21     

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