feat: highlight text in the Reader (NIP-84)

Adds NIP-84 highlights (kind 9802) to the article reader:
- Selecting text in an article shows a floating "Highlight" button
  (src/apps/articles/HighlightLayer.tsx), publishing the selected
  plain text tagged to the article (`a`) and its author (`p`, role
  "author").
- Existing highlights for the article are listed underneath it, with
  the highlighter's identity and timestamp.

Closes #23

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BYiUtZMQeA5RHggQw73wto

docs/apps.md

@@ -96,11 +96,18 @@ export default function ExampleApp({ setTitle }: AppProps) {
 | Feed | `feed` | — | kind 1 timeline, Following/Global, composer (⌘↵ publishes) |
 | Profile | `profile` | `pubkey`, `relays?` | kind 0 metadata, the author's notes, follow/unfollow |
 | Note | `notes` | `id`, `relays?` | One note and its replies. **Not** a singleton |
-| Reader | `articles` | `pubkey?`, `identifier?`, `kind?`, `relays?` | NIP-23 long-form, `react-markdown` |
+| Reader | `articles` | `pubkey?`, `identifier?`, `kind?`, `relays?` | NIP-23 long-form, `react-markdown`, NIP-84 highlights |
 | Relays | `relays` | — | Connection state, subscription count, measured latency |
 | Settings | `settings` | — | Theme, relay list, Blossom servers, account, session |
 | About | `about` | — | What this is, the app list, the shortcuts |
 
+### Highlighting selects against the DOM, not the markdown source
+
+`HighlightLayer` (`src/apps/articles/HighlightLayer.tsx`) tracks `window.getSelection()`
+against the rendered article, not the raw markdown — the highlighted text saved to a kind
+9802 event is whatever `Range.toString()` returns, i.e. the plain-text content the reader
+actually saw, not markdown syntax.
+
 ### Follow lists are a whole-list replacement
 
 kind 3 replaces the entire contact list. The follow button therefore reads the current