Hover tooltips, clickable cross-references, easier highlighting

- Tooltips on hover and keyboard focus across the app: icon-only buttons show
  their label, existing titles show in the app's style without the browser's
  delay, and the reader's main buttons show shortcut hints ("Search (/)").
  Touch screens are unaffected.
- Reader: cross-references (under verses and in the Cross-refs tab) open the
  passage's text in the side panel without leaving the chapter; "Open chapter"
  goes there when wanted
- Reader: the verse panel now has a Highlight row of colours for one-tap
  whole-verse highlighting (tap again to remove, Clear for all, with Undo);
  selecting words still highlights just those
- Fix: a cross-reference preview was cleared when React ran the panel's
  effect twice (development builds)

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

src/components/study/PageShell.jsx

@@ -83,7 +83,7 @@ export default function PageShell({ title, onBack, actions, active = null, maxWi
           </nav>
           <div className="ml-auto flex items-center gap-2">
             {openPalette && (
-              <button type="button" onClick={openPalette} aria-label="Search and go to"
+              <button type="button" onClick={openPalette} aria-label="Search and go to" data-tip="Search studies, notes and passages (⌘K)"
                 className="flex items-center gap-2 rounded-full border border-[var(--r-line)] bg-[var(--r-bg)] px-3 py-1 text-xs text-[var(--r-sub)] hover:bg-black/5">
                 <SearchIcon /><span className="hidden sm:inline">Search</span><kbd className="hidden rounded border border-[var(--r-line)] px-1 text-[10px] md:inline">⌘K</kbd>
               </button>