docs: rewrite README hero — what / who / try the demo (30-second read)

Adds a 'What it is' section between the badges and the existing 'Why
HealthLog?' paragraph so a stranger landing on the repo sees the
product proposition in under 30 seconds without scrolling. The section
gives a two-paragraph product description and a direct CTA to the
live demo, addressing the marketing-checklist gap.

Also reframes the Quick Start lead from 'Plan ~5 minutes' to
'3 minutes from git clone to a working install' so the install-time
promise lives next to the install instructions.

README.md

@@ -29,6 +29,14 @@
 
 ---
 
+## What it is
+
+HealthLog is a self-hosted personal health tracker that runs from a single `docker compose up`. It covers the metrics most people actually log -- weight, blood pressure, pulse, body composition, blood glucose, sleep, mood, and medication compliance -- and brings them together in one dashboard with reference ranges from ESC/ESH 2018, ADA 2024, and NICE NG115. Withings devices sync automatically; multi-provider AI Insights (BYOK or local) explain what the numbers mean; a doctor-report PDF generates client-side. EN/DE end-to-end. AGPL-3.0.
+
+Built for people who want their health data on their own server -- whether that's a NAS, a homelab, or a small VPS -- and who don't want to hand it to a US cloud to read a 7-day weight trend. **Try the [live demo](https://demo.healthlog.dev)** to see what a working install looks like, or skip to [Quick Start](#quick-start) below.
+
+---
+
 ## Why HealthLog?
 
 Most health apps lock your data behind proprietary clouds, push subscriptions, and sell your metrics to advertisers. HealthLog takes a different approach: your data stays on your server, encrypted at rest, accessible only to you.
@@ -71,7 +79,7 @@ Most health apps lock your data behind proprietary clouds, push subscriptions, a
 
 ## Quick Start
 
-Plan ~5 minutes for a working install. The bundled `docker-compose.yml` pulls a pre-built multi-arch image (`linux/amd64` + `linux/arm64`) from [GitHub Container Registry](https://github.com/MBombeck/HealthLog/pkgs/container/healthlog); no build step required for self-hosters. Contributors who want to test local changes can `docker compose up --build`.
+**3 minutes from `git clone` to a working install.** The bundled `docker-compose.yml` pulls a pre-built multi-arch image (`linux/amd64` + `linux/arm64`) from [GitHub Container Registry](https://github.com/MBombeck/HealthLog/pkgs/container/healthlog); no build step required for self-hosters. Contributors who want to test local changes can `docker compose up --build`.
 
 ```bash
 git clone https://github.com/MBombeck/HealthLog.git