Sync bookmarks, highlights and display preferences to the account

Bookmarks and text highlights are saved per user and merged across devices
(union on first contact, newer server copy when this device is clean). Theme,
default translation, verse layout, commentary source and reading position sync
too, each with its own timestamp so the newest change to each setting wins.
The Reader never moves under you while it is open. Signing out clears this
device's marks and forgets sync timestamps.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/pages/SettingsPage.jsx

@@ -264,8 +264,7 @@ export default function SettingsPage() {
             <h2 className="text-base font-medium text-[var(--r-text)]">My reading journal</h2>
             <p className="text-sm text-[var(--r-sub)]">
               A document of your bookmarked verses, highlighted passages, and completed reading plans.
-              Bookmarks and highlights only live on this device (they're not synced), so this reflects
-              what's saved here.
+              Bookmarks and highlights are saved to your account and follow you between devices.
             </p>
           </div>
           <div className="flex flex-wrap items-center gap-3">