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>

data/resources/style.css

@@ -66,7 +66,29 @@
     color: @window_fg_color;
 }
 
+/* ===== Drop Overlay ===== */
+.drop-overlay-scrim {
+    background: alpha(@window_bg_color, 0.75);
+}
+
+.drop-zone-card {
+    background: @card_bg_color;
+    border-radius: 16px;
+    padding: 48px 40px;
+    border: 2px dashed alpha(@accent_bg_color, 0.5);
+    box-shadow: 0 8px 32px alpha(black, 0.15);
+}
+
+.drop-zone-icon {
+    color: @accent_bg_color;
+    opacity: 0.7;
+}
+
 /* ===== Card View (using libadwaita .card) ===== */
+.card {
+    padding: 24px 20px;
+}
+
 flowboxchild:focus-visible .card {
     outline: 2px solid @accent_bg_color;
     outline-offset: 3px;