Handle requests for /.static/styles.css via top-level router

[jwodder]

Also, ensure all WebDAV responses (including errors) have "Allow" and "DAV" headers.

[codecov]

Codecov Report

Attention: 20 lines in your changes are missing coverage. Please review.

Comparison is base (7579e50) 32.78% compared to head (663e16c) 32.70%.
Report is 3 commits behind head on main.

Files	Patch %	Lines
src/dav/mod.rs	0.00%	12 Missing ⚠️
src/main.rs	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
- Coverage   32.78%   32.70%   -0.09%     
==========================================
  Files          17       17              
  Lines        2629     2636       +7     
==========================================
  Hits          862      862              
- Misses       1767     1774       +7     

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