feat: add dark mode, telemetry, and curl to Docker image

- Install `curl` in Dockerfile for healthcheck/tooling support
- Add class-based dark mode via Tailwind `@custom-variant` and a
  pre-paint `<script>` in `app.html` to avoid theme flash on load
- Implement theme toggle in layout with system preference detection,
  `localStorage` persistence, and smooth transitions
- Update all UI components with `dark:` variants for full dark mode
  support across backgrounds, borders, and text colours
- Add `sendTelemetry` helper in `api.ts` for fire-and-forget
  client-side error reporting to `/api/v1/telemetry`

Dockerfile

@@ -2,6 +2,10 @@ FROM python:3.12-slim
 
 WORKDIR /app
 
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends curl \
+    && rm -rf /var/lib/apt/lists/*
+
 COPY . .
 RUN pip install --no-cache-dir .