fix: use window_manager to set minimum window size

[Merrit]

Resolves #762
Resolves #1559

Fixes being unable to resize the tiny default window on Linux.

Having window_manager available also gives us the ability to do many window-related operations in the future if desired.

If this is merged I'd next suggest we look at the ability to remember window size & position between launches.

[CLAassistant]

All committers have signed the CLA.

[LucasXu0]

windowManager.setMinimumSize(const Size(minWidth, minWidth / ratio)); will cause a crash on macOS.

[LucasXu0]

I have addressed the problem. The width and height should be integers in this case.

[codecov-commenter]

Codecov Report

Merging #1791 (50ead3f) into main (84ea888) will decrease coverage by 21.84%.
The diff coverage is n/a.

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@@             Coverage Diff             @@
##             main    #1791       +/-   ##
===========================================
- Coverage   38.01%   16.18%   -21.84%     
===========================================
  Files         423      301      -122     
  Lines       19882    12966     -6916     
===========================================
- Hits         7559     2098     -5461     
+ Misses      12323    10868     -1455     

Flag	Coverage Δ
appflowy	16.18% <ø> (-0.01%)	⬇️
appflowy_editor	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ontend/app_flowy/lib/startup/tasks/app_widget.dart	85.71% <ø> (-0.40%)	⬇️
..._editor/lib/src/render/rich_text/heading_text.dart
...wy_editor/lib/src/commands/text/text_commands.dart
...lowy_editor/lib/src/service/selection_service.dart
...editor/lib/src/render/color_menu/color_picker.dart
...tor/lib/src/service/context_menu/context_menu.dart
.../appflowy_editor/lib/src/infra/html_converter.dart
...ns/markdown/decoder/document_markdown_decoder.dart
...owy_editor/lib/src/render/rich_text/rich_text.dart
...ppflowy_editor/lib/src/service/scroll_service.dart
... and 113 more

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