docs(user): web reader guide — navigation, touch selection, highlights

User-facing documentation for the rebuilt web reader (§6.5 of the app
handoff): swipe-to-page / tap-for-menu touch navigation, long-press word
selection with drag extension and handles, the selection popover (color
dots, notes with save-together-on-create, copy incl. plain-HTTP LAN,
edit/delete by tapping a painted highlight), below-the-selection popover
placement rationale, PDF/comic behavior, and annotations drawer.
Linked from the user documentation portal and the docs index quick
links / quick-find tables.

docs/index.md

@@ -77,6 +77,7 @@ Complete guide to Bookhoard documentation. Find what you need quickly.
 | **Deploy Bookhoard** | [Operations Portal → Troubleshooting](operations/troubleshooting.md)   |
 | **Contribute code**  | [Contributing Portal → Development Guide](contributing/development.md) |
 | **Understand sync**  | [User Portal → Sync Guide](user/sync-guide.md)                         |
+| **Read in the browser** | [User Portal → Web Reader Guide](user/reader-guide.md)              |
 
 ---
 
@@ -90,6 +91,7 @@ Complete guide to Bookhoard documentation. Find what you need quickly.
 | ...set up KOReader?         | [KOReader Setup Guide](user/devices/koreader-setup.md) |
 | ...use a Kobo?              | [Kobo Setup Guide](user/devices/kobo-setup.md) - native sync coming soon; KOReader works today |
 | ...understand sync?         | [Sync Guide](user/sync-guide.md)                       |
+| ...read in the browser?     | [Web Reader Guide](user/reader-guide.md) - reading, highlights, notes |
 | ...resolve conflicts?       | [Sync Guide](user/sync-guide.md) - Managing Conflicts  |
 | ...troubleshoot deployment? | [Troubleshooting Guide](operations/troubleshooting.md) |
 | ...use the API?             | [API Reference](developer/api-reference.md)            |