feat: add SFTP host key management and security improvements

- Add HTTPS URL validation for extension store registry and downloads
- Add Reset SFTP Host Key button (per-server)
- Add Reset All SFTP Host Keys button
- Add SFTP host key verification with TOFU (Trust On First Use)
- Update cloud upload service with host key storage
- Add flutter_secure_storage dependency for secure password storage

pubspec.lock

@@ -451,6 +451,54 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.1.0"
+  flutter_secure_storage:
+    dependency: "direct main"
+    description:
+      name: flutter_secure_storage
+      sha256: "9cad52d75ebc511adfae3d447d5d13da15a55a92c9410e50f67335b6d21d16ea"
+      url: "https://pub.dev"
+    source: hosted
+    version: "9.2.4"
+  flutter_secure_storage_linux:
+    dependency: transitive
+    description:
+      name: flutter_secure_storage_linux
+      sha256: be76c1d24a97d0b98f8b54bce6b481a380a6590df992d0098f868ad54dc8f688
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.2.3"
+  flutter_secure_storage_macos:
+    dependency: transitive
+    description:
+      name: flutter_secure_storage_macos
+      sha256: "6c0a2795a2d1de26ae202a0d78527d163f4acbb11cde4c75c670f3a0fc064247"
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.1.3"
+  flutter_secure_storage_platform_interface:
+    dependency: transitive
+    description:
+      name: flutter_secure_storage_platform_interface
+      sha256: cf91ad32ce5adef6fba4d736a542baca9daf3beac4db2d04be350b87f69ac4a8
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.1.2"
+  flutter_secure_storage_web:
+    dependency: transitive
+    description:
+      name: flutter_secure_storage_web
+      sha256: f4ebff989b4f07b2656fb16b47852c0aab9fed9b4ec1c70103368337bc1886a9
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.2.1"
+  flutter_secure_storage_windows:
+    dependency: transitive
+    description:
+      name: flutter_secure_storage_windows
+      sha256: b20b07cb5ed4ed74fc567b78a72936203f587eba460af1df11281c9326cd3709
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.1.2"
   flutter_svg:
     dependency: "direct main"
     description:
@@ -561,10 +609,10 @@ packages:
     dependency: transitive
     description:
       name: js
-      sha256: "53385261521cc4a0c4658fd0ad07a7d14591cf8fc33abbceae306ddb974888dc"
+      sha256: f2c445dce49627136094980615a031419f7f3eb393237e4ecd97ac15dea343f3
       url: "https://pub.dev"
     source: hosted
-    version: "0.7.2"
+    version: "0.6.7"
   json_annotation:
     dependency: "direct main"
     description: