feat(desktop): redesign Settings + fix unreadable bubble text

Two UI fixes:

1. MessageBubble: 'Nachricht nicht lesbar' was rendered with
   text-fg-muted on the blue 'mine' bubble — invisible. Now uses
   text-accent-fg/80 on mine, text-fg-muted on peer (still
   ≥4.5:1 contrast in both modes).

2. SettingsPage: redesigned from a long single-column scroll into a
   sticky-sidebar + content layout (lg+) with:
   - 9 anchor-linked sections with icons in the sidebar
   - IntersectionObserver highlights the active section
   - Each section has a description subtitle for context
   - Voice (the densest section) is now sub-grouped into Audio-Gerät /
     Qualität / PTT / Hotkeys / E2EE via SubSection cards
   - Notifications consolidates message-sound + ringtone
   - Danger-toned account section visually separated
   - Mobile fallback is the original single-column scroll

apps/desktop/src/components/MessageBubble.tsx

@@ -416,7 +416,14 @@ export function MessageBubble({
               </button>
             )}
             {message.plaintext === null ? (
-              <span className="italic text-fg-muted opacity-60">
+              <span
+                className={
+                  'italic ' +
+                  // Mine = blue/accent bubble → use accent-fg with reduced opacity
+                  // (still meets 4.5:1). Peer = surface-2 grey → muted-fg works.
+                  (mine ? 'text-accent-fg/80' : 'text-fg-muted')
+                }
+              >
                 {t('app:chats.unreadable', {
                   defaultValue: 'Nachricht nicht lesbar',
                 })}