Fix selectors overlapping bottom toolbars

[amantoux]

Color and heading selectors can cover the formatting toolbar when it sits above the keyboard. Position selectors below the presenter when there is room, and flip above it when that side offers more space. Use the full toolbar's vertical bounds while keeping horizontal alignment with the button, and preserve keyboard and safe-area constraints.

[codecov]

Codecov Report

❌ Patch coverage is 97.08405% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.11%. Comparing base (0dbf67a) to head (836e461).

Files with missing lines	Patch %	Lines
packages/parchment/lib/src/document/embeds.dart	50.00%	4 Missing ⚠️
packages/parchment/lib/src/codecs/html_utils.dart	25.00%	3 Missing ⚠️
...ackages/parchment/lib/src/document/attributes.dart	66.66%	3 Missing ⚠️
packages/parchment/lib/src/codecs/markdown.dart	95.55%	2 Missing ⚠️
packages/parchment/lib/src/document/leaf.dart	66.66%	2 Missing ⚠️
...kages/fleather/lib/src/widgets/editor_toolbar.dart	96.66%	1 Missing ⚠️
packages/parchment/lib/src/document.dart	90.90%	1 Missing ⚠️
packages/parchment/lib/src/document/line.dart	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #590      +/-   ##
==========================================
+ Coverage   88.08%   88.11%   +0.02%     
==========================================
  Files          63       63              
  Lines       10903    10985      +82     
==========================================
+ Hits         9604     9679      +75     
- Misses       1299     1306       +7     

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