Web dashboard step 7: docs, users page, polish

Users page, README and config.example updates, implementation notes, the
rollout runbook, site-layout 404/500 pages, human-readable download sizes,
dark-mode and narrow-screen polish, and a smoke test over every dashboard
route; also removes zdiff3 ancestor markers left by earlier merges.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NHyYupFdBiR4VfoUM7NjSM

src/web/templates/issue_full.html

@@ -4,7 +4,7 @@
 <hr class="rule">
 <h1 class="kicker">The Brief</h1>
 <div class="editorial">{{ front_page_html|safe }}</div>
-{% if !downloads.is_empty() %}<p class="downloads">{% for download in downloads %}<a class="button" href="{{ download.href }}">Download {{ download.label }} <small>({{ download.size_bytes }} bytes)</small></a>{% endfor %}</p>{% endif %}
+{% if !downloads.is_empty() %}<p class="downloads">{% for download in downloads %}<a class="button" href="{{ download.href }}">Download {{ download.label }} <small>({{ download.size }})</small></a>{% endfor %}</p>{% endif %}
 <hr class="rule">
 <h1>In This Issue</h1>
 {% for section in sections %}<section><h2>{{ section.name }}</h2><ul class="index-list">{% for entry in section.entries %}<li class="index-entry">