feat: implement native OS keyring for site logins

- Replaced plaintext local storage password fields with keyring v3 OS native integration.
- Tauri backend securely sets, gets, and deletes credentials via system keychains.
- Dynamic password injection occurs seamlessly on download start and metadata parsing.

apps/desktop/src-tauri/Cargo.lock

@@ -1949,6 +1949,16 @@ dependencies = [
  "unicode-segmentation",
 ]
 
+[[package]]
+name = "keyring"
+version = "3.6.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "eebcc3aff044e5944a8fbaf69eb277d11986064cba30c468730e8b9909fb551c"
+dependencies = [
+ "log",
+ "zeroize",
+]
+
 [[package]]
 name = "leb128fmt"
 version = "0.1.0"
@@ -3813,6 +3823,7 @@ dependencies = [
 name = "tauri-app"
 version = "0.1.0"
 dependencies = [
+ "keyring",
  "regex",
  "reqwest 0.12.28",
  "serde",