Add iOS install hint component

InstallHint shows "Tap Share, then Add to Home Screen to get notifications"
only in iOS and iPadOS Safari browser tabs (not when display-mode is
standalone or navigator.standalone is true), since iOS delivers Web Push
only to installed apps. Dismissing it is remembered per device in
localStorage; when storage is unavailable the hint still shows and
dismisses for the page's lifetime. Exported from src/pwa/InstallHint.tsx
for onboarding step 3 and the Notifications screen.

Maestro task: ad6b3d80-9f41-40ac-8e2a-f786e1fa9777

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

web/src/pwa/InstallHint.module.css

@@ -0,0 +1,18 @@
+.hint {
+  display: flex;
+  align-items: center;
+  gap: var(--space-3);
+}
+
+.icon {
+  color: var(--color-accent);
+}
+
+.message {
+  flex: 1;
+  margin: 0;
+  font-family: var(--font-body);
+  font-size: 15px;
+  font-weight: 600;
+  line-height: 1.35;
+}