fix(core,fuse,gui): UI/UX quick wins from the audit (phase 0)

- Move Google Takeout import out of Settings; Photos owns it, and the
  Takeout page's back button returns there.
- Trash from the file browser is Undo-only (no confirm); F2 on a
  multi-selection explains why it does nothing.
- Ask before sign-out, rejecting an invitation, removing someone's access,
  removing a public link, and switching a folder to on-demand.
- Clearer offline wording ("Make available offline" / "Make online only")
  with dedicated offline, online-only and upload symbolic icons; the pin row
  is hidden for folders.
- Search hits report whether a file is cached, so search results show the
  offline badge.
- Activity log: readable Empty Trash / link entries, distinct icons, and
  error/warning styling for failures and conflicts.
- Proper singular/plural counts instead of "item(s)" / "archive(s)".
- Name earlier versions by save time instead of revision id; capitalise
  the public link role; stop-syncing uses a stop icon, not a trash can.
- Label the Photos Upload and "Show on map" buttons; toast when a
  thumbnail build finishes; the versions button follows the mount state.
- Mode switches and device mutations reload both sync pages.

crates/pdfs-gui/resources/pdfs.gresource.xml

@@ -6,5 +6,8 @@
   <gresource prefix="/de/nils/protondrivelinux/icons">
     <file preprocess="xml-stripblanks">scalable/actions/pdfs-build-thumbnails-symbolic.svg</file>
     <file preprocess="xml-stripblanks">scalable/actions/pdfs-folder-upload-symbolic.svg</file>
+    <file preprocess="xml-stripblanks">scalable/actions/pdfs-upload-symbolic.svg</file>
+    <file preprocess="xml-stripblanks">scalable/actions/pdfs-offline-symbolic.svg</file>
+    <file preprocess="xml-stripblanks">scalable/actions/pdfs-online-only-symbolic.svg</file>
   </gresource>
 </gresources>