Add launch-time notification permission prompt and UI polish

- Request POST_NOTIFICATIONS once on first app launch, with a
  plain-language rationale dialog shown before the OS prompt
  (permission_handler), since upload/download progress relies on it.
- ShareUploadView no longer shows "More tabs" - there's nowhere useful
  for it to go mid-upload.
- FloatingBottomNavBar's pill scrolls horizontally instead of
  overflowing when the visible tab set doesn't fit.
- Settings account card: drop the separate server-address row and its
  globe icon, showing the host inline with the quota percentage/
  "Unlimited storage" text instead (left-aligned, same size).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

pubspec.yaml

@@ -55,6 +55,7 @@ dependencies:
   marquee: ^2.3.0
   local_auth: ^2.3.0
   webview_flutter: ^4.14.1
+  permission_handler: ^11.3.1
 
 dev_dependencies:
   flutter_test: