docs(build): switch from html to dirhtml builder for clean URLs

why: The html builder generates relative paths like ../index.html and #
that go stale after SPA navigation across directory depths. The dirhtml
builder generates clean URLs natively (/api/ instead of /api/index.html).
what:
- Change html, start, and design recipes to use -b dirhtml
- sphinx-autobuild serves dirhtml correctly (StaticFiles html=True)
- SPA nav already handles / endings (shouldIntercept checks endsWith("/"))
- rediraffe supports dirhtml natively (DirectoryHTMLBuilder detection)
- No changes needed to redirects.txt, conf.py, or documentation source

Author: tony

docs/justfile

@@ -23,7 +23,7 @@ default:
 # Build HTML documentation
 [group: 'build']
 html:
-    {{ sphinxbuild }} -b html {{ allsphinxopts }} {{ builddir }}/html
+    {{ sphinxbuild }} -b dirhtml {{ allsphinxopts }} {{ builddir }}/html
     @echo ""
     @echo "Build finished. The HTML pages are in {{ builddir }}/html."
 
@@ -198,9 +198,9 @@ dev:
 # Start sphinx-autobuild server
 [group: 'dev']
 start:
-    uv run sphinx-autobuild "{{ sourcedir }}" "{{ builddir }}" {{ sphinxopts }} --port {{ http_port }}
+    uv run sphinx-autobuild -b dirhtml "{{ sourcedir }}" "{{ builddir }}" {{ sphinxopts }} --port {{ http_port }}
 
 # Design mode: watch static files and disable incremental builds
 [group: 'dev']
 design:
-    uv run sphinx-autobuild "{{ sourcedir }}" "{{ builddir }}" {{ sphinxopts }} --port {{ http_port }} --watch "." -a
+    uv run sphinx-autobuild -b dirhtml "{{ sourcedir }}" "{{ builddir }}" {{ sphinxopts }} --port {{ http_port }} --watch "." -a