Web dashboard v2 step 2: Tailwind design system, theme toggle, reader pages

Replaces the hand-written stylesheet with a Tailwind CSS v4 source
(src/web/tailwind.css) whose minified build is committed as
src/web/static/app.css and still embedded with include_str!, so the binary
and the test suite never need Node. package.json pins tailwindcss and
@tailwindcss/cli to 4.3.3 and adds css, css:watch and css:check.

Colours are semantic CSS variables swapped per theme and exposed to Tailwind
through @theme inline, so templates write bg-paper/text-ink/border-rule once
and both themes work. theme.js runs synchronously in <head> to set
data-theme before paint; app.js cycles the toggle system -> light -> dark and
persists the choice. Newsreader is self-hosted and served from
web::static_asset with the same ETag/304 logic as the other assets.

Every reader template is restyled: ears row, centred masthead with its rules,
tracked section labels, the drop cap on The Brief, "Why it's here" with its
accent bar, the segmented rating control, prev/next cards, and the colophon
as a two-column definition grid. Dashboard templates are untouched (step 3).

Also fixes a .block collision with Tailwind's display utility, the SVG
`hidden` property that never toggled the theme icon, and points the dev
seed's profile_path at ./dev so previewing cannot rewrite data/profile.md.

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

src/web/templates/world.html

@@ -1,5 +1 @@
-{% extends "layout.html" %}{% block content %}<article class="reading world-briefing">
-<h1>World Briefing</h1><p class="dateline">{{ display_date }}</p><hr class="rule">
-<div class="world-body">{{ body_html|safe }}</div>
-<p><a href="{{ issue_href }}">← Back to this issue</a></p>
-</article>{% endblock %}
+{% extends "layout.html" %}{% block ears %}<span>{{ display_date }}</span>{% endblock %}{% block content %}<article class="mx-auto mt-10 max-w-[68ch] px-4 sm:mt-14 sm:px-6"><header class="mb-9"><p class="font-sans text-[0.72rem] uppercase tracking-[0.12em] text-muted">{{ display_date }}</p><h1 class="mt-2 text-4xl font-semibold leading-[1.1] tracking-[-0.01em] sm:text-5xl">World Briefing</h1></header><div class="prose-body border-t border-rule pt-7">{{ body_html|safe }}</div><p class="mt-10 border-t border-rule pt-5 font-sans text-sm"><a href="{{ issue_href }}">← Back to this issue</a></p></article>{% endblock %}