fix(sync): enable keyring windows-native backend — creds were a silent no-op

keyring 3.x feature-gates its OS backends OFF by default, so with no feature the
credential store fell back to a mock where set/get don't share state: every
password write appeared to succeed but read back empty, so no Basic auth was
sent and WebDAV/Nextcloud auth always 401'd. windows-native pulls the real
Windows Credential Manager. Also affected MCP/hosted-AI keys + OAuth tokens.

src-tauri/Cargo.lock

@@ -2209,7 +2209,9 @@ version = "3.6.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "eebcc3aff044e5944a8fbaf69eb277d11986064cba30c468730e8b9909fb551c"
 dependencies = [
+ "byteorder",
  "log",
+ "windows-sys 0.60.2",
  "zeroize",
 ]
 
@@ -5973,7 +5975,7 @@ dependencies = [
 
 [[package]]
 name = "whispassist"
-version = "0.1.5"
+version = "0.1.6"
 dependencies = [
  "argon2",
  "async-trait",

src-tauri/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "whispassist"
-version = "0.1.5"
+version = "0.1.6"
 description = "Privacy-first, fully local Windows meeting assistant"
 authors = ["WhispAssist contributors"]
 license = "MIT OR Apache-2.0"
@@ -43,7 +43,7 @@ argon2 = "0.5"
 chacha20poly1305 = "0.10"
 getrandom = "0.2"
 zeroize = "1"                                               # wipe key material from memory on lock
-keyring = { version = "3", optional = true }               # OS credential store (sync + AI creds)
+keyring = { version = "3", optional = true, features = ["windows-native"] }  # OS credential store (sync + AI creds); windows-native = real Credential Manager (else keyring 3.x uses a no-op mock store)
 rmcp = { version = "0.16", optional = true, features = ["server"] }  # MCP server (ADR-0011)
 
 # audio / transcription / diarization / calendar are integrated per-phase and are