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/icons/scalable/actions/pdfs-offline-symbolic.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
+  <!-- "Available offline": a filled disc with a down arrow knocked out of it
+       (one path, fill-rule evenodd, so symbolic recolouring keeps working).
+       Replaces the star, which every desktop uses for "favourite". -->
+  <path fill="#222222" fill-rule="evenodd" d="M 8,1 A 7,7 0 1 0 8,15 A 7,7 0 1 0 8,1 Z M 7,4 H 9 V 8.6 L 10.8,6.8 L 12.2,8.2 L 8,12.4 L 3.8,8.2 L 5.2,6.8 L 7,8.6 Z"/>
+</svg>

crates/pdfs-gui/resources/icons/scalable/actions/pdfs-online-only-symbolic.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
+  <!-- "Online only": a cloud outline. Lighter than the stock weather icon, so a
+       folder full of cloud-only files does not shout. -->
+  <path fill="#222222" fill-rule="evenodd" d="M 8.5,3 C 6.6,3 5,4.3 4.6,6.1 C 2.6,6.3 1,8 1,10 C 1,12.2 2.8,14 5,14 H 12 C 13.7,14 15,12.7 15,11 C 15,9.5 13.9,8.2 12.4,8 C 12.1,5.2 10.5,3 8.5,3 Z M 8.5,4.5 C 9.9,4.5 11,5.9 11,8 V 9.4 H 12 C 12.8,9.4 13.5,10.1 13.5,11 C 13.5,11.8 12.8,12.5 12,12.5 H 5 C 3.6,12.5 2.5,11.4 2.5,10 C 2.5,8.6 3.6,7.5 5,7.5 H 5.9 L 6,6.7 C 6.2,5.4 7.2,4.5 8.5,4.5 Z"/>
+</svg>

crates/pdfs-gui/resources/icons/scalable/actions/pdfs-upload-symbolic.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
+  <!-- Upload-files action: an up arrow over a tray, the pair to
+       pdfs-folder-upload-symbolic. Stock themes only offer `document-send`,
+       which reads as "send by mail". -->
+  <path fill="#222222" d="M 8,1 L 3.5,5.5 L 4.9,6.9 L 7,4.8 V 11 H 9 V 4.8 L 11.1,6.9 L 12.5,5.5 Z"/>
+  <path fill="#222222" d="M 1,10 V 14 C 1,14.6 1.4,15 2,15 H 14 C 14.6,15 15,14.6 15,14 V 10 H 13 V 13 H 3 V 10 Z"/>
+</svg>