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

src/core/database.rs

@@ -703,7 +703,17 @@ impl Database {
                 last_scanned = datetime('now')",
             params![path, filename, appimage_type, size_bytes, is_executable, file_modified],
         )?;
-        Ok(self.conn.last_insert_rowid())
+        // last_insert_rowid() returns 0 for ON CONFLICT UPDATE, so query the actual id
+        let id = self.conn.last_insert_rowid();
+        if id != 0 {
+            return Ok(id);
+        }
+        let id: i64 = self.conn.query_row(
+            "SELECT id FROM appimages WHERE path = ?1",
+            params![path],
+            |row| row.get(0),
+        )?;
+        Ok(id)
     }
 
     pub fn update_metadata(