Fix icon extraction, drop overlay styling, and card padding

- Fix upsert_appimage returning 0 for existing records by falling
  back to a SELECT query when last_insert_rowid is 0
- Replace --appimage-offset execution with binary squashfs magic scan
  to avoid hanging on AppImages with custom AppRun scripts
- Add 5-second timeout fallback if binary scan fails
- Extract desktop files from usr/share/applications/ for reverse-DNS
  named entries that root-level *.desktop glob misses
- Add root-level png/svg fallback in icon search
- Add CSS for drop overlay scrim, drop zone card, and drop zone icon
- Add card padding (24px 20px) so content does not touch card edges
- Always scan on startup to discover new AppImages

data/app.driftwood.Driftwood.gschema.xml

@@ -32,7 +32,7 @@
       <description>Application color scheme: default (follow system), force-light, or force-dark.</description>
     </key>
     <key name="auto-scan-on-startup" type="b">
-      <default>false</default>
+      <default>true</default>
       <summary>Auto scan on startup</summary>
       <description>Whether to automatically scan for AppImages when the application starts.</description>
     </key>