Refresh screenshot galleries and repair portal upgrades

Use the September 30 hero on the website and README, add a shared screenshot gallery, and remove the old showcase images. Build image links locally and clean generated site output.

Back up the known development portal override during installation and report custom overrides before building. Keep source selections clear while letting blur show through unselected rows.

scripts/tests/test_portal_install.py

@@ -57,6 +57,41 @@ class PortalInstallTest(unittest.TestCase):
                                        'replace_portal_config': str(replace).lower()},
                                   capture_output=True, text=True)
 
+    def user_override(self, content, backup=False):
+        script = (REPO / 'scripts/install.sh').read_text()
+        helpers = script[script.index('check_user_portal_override()'):script.index('fail() {')]
+        with tempfile.TemporaryDirectory() as directory:
+            root = Path(directory)
+            override = root / 'override.conf'
+            if content is not None:
+                override.write_text(content.replace('USER_HOME', directory))
+            command = 'backup_user_portal_override' if backup else 'check_user_portal_override'
+            result = subprocess.run(['bash', '-eu', '-c', helpers +
+                                     f'\n{command} "$TEST_ROOT/override.conf" "$TEST_ROOT" test-release'],
+                                    env={'TEST_ROOT': directory}, capture_output=True, text=True)
+            saved = root / 'override.conf.before-test-release'
+            return result, override.exists(), saved.read_text() if saved.exists() else None
+
+    def test_user_development_override_is_backed_up(self):
+        content = '[Service]\nExecStart=\nExecStart=USER_HOME/.local/libexec/ferese/xdg-desktop-portal-ferese\n'
+        result, exists, saved = self.user_override(content, backup=True)
+        self.assertEqual(result.returncode, 0, result.stderr)
+        self.assertFalse(exists)
+        self.assertIn('/.local/libexec/ferese/xdg-desktop-portal-ferese', saved)
+
+    def test_custom_user_override_is_preserved_and_reported(self):
+        result, exists, saved = self.user_override('[Service]\nExecStart=/custom/portal\n', backup=True)
+        self.assertNotEqual(result.returncode, 0)
+        self.assertTrue(exists)
+        self.assertIsNone(saved)
+        self.assertIn('takes precedence', result.stderr)
+
+    def test_missing_user_override_needs_no_backup(self):
+        result, exists, saved = self.user_override(None, backup=True)
+        self.assertEqual(result.returncode, 0, result.stderr)
+        self.assertFalse(exists)
+        self.assertIsNone(saved)
+
     def test_existing_development_install_requires_explicit_replacement(self):
         portal = ('[portal]\nDBusName=org.freedesktop.impl.portal.desktop.ferese\n'
                   'Interfaces=org.freedesktop.impl.portal.ScreenCast;\nUseIn=Ferese;\n')