feat: add emoji reactions, light/dark themes and rollout observability

Add a set of guest-facing and operator-facing features that make the
deployment-strategy demos (recreate, rolling, canary) more visual:

- Emoji reactions (heart/tada/thumbsup/smile): visitors react without an
  account, a first-party cookie keeps counts honest, the tally streams
  live to every tab over SSE. Backed by a new reactions table with an
  idempotent upsert; whitelisted emoji so the endpoint can't be abused.
- Light & dark themes: the wall follows the OS preference, visitors can
  toggle it (persisted locally), and a moderator can force a theme for
  everyone from the admin console, pushed live over SSE.
- Per-replica request counter: a middleware tallies requests answered by
  each replica; surfaced in /healthz, /api/info and the admin replica
  table. Under load-balancing the counts diverge, and a rolling update
  resets them one container at a time — the rollout made visible.
- Rolling-update wave in the Chaos lab: bounces replicas one at a time
  (via the crash endpoint) while polling the replica table, so a training
  session can watch each replica reset and rejoin — what a rolling update
  looks like from the inside.
- Admin console: reactions tile, per-replica Requests column, reaction
  tallies in moderation, and a wall-theme setting.
- Favicon + /favicon.ico redirect.

Tests cover reactions (idempotency, tallies, whitelist) and the theme
setting round-trip.

Akili-Task: tsk_xgg6sfuzkzymyc6gstr4
Akili-Agent: goldfish

web/badges/icon.svg

@@ -0,0 +1,11 @@
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+  <defs>
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+    </linearGradient>
+  </defs>
+  <rect width="64" height="64" rx="14" fill="url(#g)"/>
+  <path d="M20 34h24v4a8 8 0 0 1-8 8h-8a8 8 0 0 1-8-8v-4z" fill="#fff" opacity=".92"/>
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+</svg>