Store encryption key alongside database for better portability

When using custom storage or after system format, the encryption key
was lost because it was only stored in GSettings. Now the key is also
saved as a .key file next to clipboard.json, so encrypted data can be
recovered even after reinstalling the OS.

Changes:
- Database.js now loads key from .key file first, then falls back to
  GSettings for backward compatibility
- Export now includes the .key file alongside clipboard.json
- Import now restores the .key file and updates GSettings

This ensures your clipboard history survives system migrations when
using custom storage locations like cloud-synced folders.

Author: sfnemis

clipmaster@gnome.extension/metadata.json

@@ -2,13 +2,18 @@
     "uuid": "clipmaster@gnome.extension",
     "name": "ClipMaster",
     "description": "ClipMaster is a clipboard manager with history, image support, favorites, and optional themes. Follows system dark/light preference by default.",
-    "version": 2,
-    "shell-version": ["45", "46", "47", "48", "49"],
+    "version": 3,
+    "shell-version": [
+        "45",
+        "46",
+        "47",
+        "48",
+        "49"
+    ],
     "url": "https://github.com/sfnemis/ClipMaster",
     "settings-schema": "org.gnome.shell.extensions.clipmaster",
     "gettext-domain": "clipmaster",
     "donations": {
         "github": "sfnemis"
     }
-}
-
+}