RTD dark canvas, favicon, scale/preset sizing, update-now fix #274

Job	Run time
Workflow lint (actionlint)	30s
Tests (windows-latest)	49s
Changelog fragment gate (Towncrier)	4s
Type Check	15s
Docs (build + linkcheck)	53s
Docs touch gate	7s
Briefcase smoke test (Linux)	48s
News fragment naming (ID + KAC type)	13s
Lint	11s
Tests (macos-latest)	55s
Prose lint (codespell + doc8 + interrogate)	20s
Dependency audit (uv audit)	9s
Tests (ubuntu-latest)	41s
	5m 55s

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