feat(gui): rewrite the search launcher with a preview pane

The launcher (pdfs-prompt) now follows the app's theme: the theme code
moved into a shared theme.rs, the prompt's stylesheet lives in
resources/prompt.css on the libadwaita palette, the Proton-purple accent
pin is gone, and every activation re-reads the Proton theme and open_with
settings so changes in the app apply without restarting the launcher.

The window is rebuilt in src/hud/:
- one list with Recent, Pinned and Best matches sections; a click
  selects, a double-click or Enter opens, focus stays in the entry
- rows show a thumbnail, the name with its matched characters in bold,
  location, sync badge, size and age
- a preview pane with a large thumbnail, type, size, modified date,
  location and availability, plus Open (Enter), Show in Folder
  (Ctrl+Enter, via FileManager1) and Copy Path (Ctrl+C / Ctrl+Shift+C)
- thumbnails from the freedesktop cache, local decoding (written back
  to the shared cache) or the daemon, on two worker threads
- a history of opened files that fills the Recent section and adds a
  frecency bonus to search ranking
- the mount check before opening a Drive file runs off the main thread
  with a 2 s timeout, and Escape cancels a slow open
- rows can be dragged out as files
- the search debounce drops to 80 ms

New strings are translated in every catalog.

crates/pdfs-gui/build.rs

@@ -27,5 +27,6 @@ fn main() {
     println!("cargo:rerun-if-changed=resources/icons");
     println!("cargo:rerun-if-changed=resources/style.css");
     println!("cargo:rerun-if-changed=resources/proton-theme.css");
+    println!("cargo:rerun-if-changed=resources/prompt.css");
     println!("cargo:rerun-if-changed=../../packaging/io.narl.proton-drive-linux.svg");
 }