Default KGet's download folder to Downloads and add release workflow

install.sh now sets KGet's remembered "New Download" folder
(kgetrc Internal/LastDirectory) to the XDG downloads directory if it
isn't already configured, since KGet's own out-of-the-box default was
Documents. Falls back correctly even when xdg-user-dir has no
user-dirs.dirs config to read (it silently returns bare $HOME
otherwise).

Also add a GitHub Actions workflow that packages extension/ into a zip
and publishes a GitHub Release with generated notes whenever a
v*.*.* tag is pushed.

.github/workflows/release.yml

@@ -0,0 +1,27 @@
+name: Release
+
+on:
+  push:
+    tags:
+      - "v*.*.*"
+
+permissions:
+  contents: write
+
+jobs:
+  release:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Package extension
+        run: |
+          cd extension
+          zip -r "../kget-capture-${GITHUB_REF_NAME}.zip" .
+
+      - name: Create release
+        uses: softprops/action-gh-release@v2
+        with:
+          files: kget-capture-${{ github.ref_name }}.zip
+          generate_release_notes: true