feat: highlight text in the Reader (NIP-84) (#31)

* 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

* fix: harden selection handling and clamp the highlight button

Per review:
- Guard sel.rangeCount === 0 before calling getRangeAt(0), which
  throws otherwise.
- Scope containment by the range's commonAncestorContainer instead of
  just anchorNode, so a selection that starts inside the article but
  is dragged out past its boundary is correctly rejected.
- Clamp the floating button's top so a selection near the top of the
  viewport doesn't push it off-screen.

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

* fix: don't highlight against a malformed address, gate the listener

Per a "needs a closer look" review pass:
- articles/index.tsx now passes an empty string, not a malformed
  "kind:pubkey:" address, when an article has no d tag. HighlightLayer
  treats a falsy address as "highlighting isn't available here."
- The selectionchange listener is only registered when both user and
  address are present (in the effect's deps), instead of always
  running selection tracking regardless of whether a highlight could
  ever be published.
- handleHighlight and the floating button both guard on address too,
  not just selection, so stale selection state from before a prop
  change went missing can't still trigger a publish.
- docs/apps.md corrected: the saved text comes from Selection.toString()
  (window.getSelection()), not Range.toString().

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

---------

Co-authored-by: highperfocused <highperfocused@pm.me>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

docs/apps.md

@@ -96,12 +96,19 @@ 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, or a blank local draft when `id` is absent. **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 |
 | Bookmarks | `bookmarks` | — | NIP-51 kind 10003 list — bookmarked notes and articles |
 | 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 that `Selection`'s `.toString()` returns, i.e. the plain-text content
+the reader actually saw, not markdown syntax.
+
 ### Bookmarks are one whole-list replacement, like follow lists
 
 kind 10003 is a replaceable event: publishing it replaces the entire list. `useToggleBookmark`