security: fix all review findings, bump to 1.0.19 (build 20)

CRITICAL
- SftpProvider: replace PromiscuousVerifier with TofuHostKeyVerifier
  (trust-on-first-use; stores SHA-256 fingerprints in EncryptedSharedPreferences;
  rejects key changes on subsequent connections)

HIGH
- GoogleDriveProvider: replace raw string interpolation with buildJsonObject
  in uploadFile, createDirectory, and moveFile to prevent JSON injection
- DropboxProvider: replace all raw JSON strings and Dropbox-API-Arg headers
  with buildJsonObject for the same reason
- OAuthHelper: add cryptographically random state parameter to Dropbox and
  OneDrive authorization URLs (stored alongside the PKCE verifier)
- OAuthRedirectActivity: validate returned state against stored value before
  exchanging the authorization code (CSRF protection)

MEDIUM
- WebDavProvider: block cross-host redirects in the manual redirect interceptor
  so Authorization headers are never forwarded to a different server
- AccountSetupScreen: set FLAG_SECURE on the window while credential fields
  are visible to prevent screenshots and screen-recording capture
- libs.versions.toml: security-crypto alpha06 → stable 1.0.0;
  biometric-ktx alpha05 → biometric 1.1.0 (stable, non-ktx artifact matches
  the BiometricManager/BiometricPrompt API actually used in MainActivity)
- CredentialStore: migrate to security-crypto 1.0.0 API (MasterKeys.getOrCreate
  + positional create() args); add saveHostKey/getHostFingerprint for SFTP TOFU

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/src/main/kotlin/com/syncflow/ui/auth/OAuthHelper.kt

@@ -38,7 +38,9 @@ private fun generateChallenge(verifier: String): String {
 
 fun launchDropboxOAuth(context: Context, credentialStore: CredentialStore, appKey: String) {
     val verifier = generateVerifier()
+    val state = generateVerifier()
     credentialStore.savePkceVerifier("dropbox", verifier)
+    credentialStore.savePkceVerifier("dropbox_state", state)
     val challenge = generateChallenge(verifier)
     val url = "https://www.dropbox.com/oauth2/authorize" +
         "?client_id=$appKey" +
@@ -46,13 +48,16 @@ fun launchDropboxOAuth(context: Context, credentialStore: CredentialStore, appKe
         "&redirect_uri=syncflow%3A%2F%2Foauth%2Fdropbox" +
         "&code_challenge=$challenge" +
         "&code_challenge_method=S256" +
-        "&token_access_type=offline"
+        "&token_access_type=offline" +
+        "&state=$state"
     openCustomTab(context, url)
 }
 
 fun launchOneDriveOAuth(context: Context, credentialStore: CredentialStore, clientId: String) {
     val verifier = generateVerifier()
+    val state = generateVerifier()
     credentialStore.savePkceVerifier("onedrive", verifier)
+    credentialStore.savePkceVerifier("onedrive_state", state)
     val challenge = generateChallenge(verifier)
     val scopes = "Files.ReadWrite+User.Read+offline_access"
     val url = "https://login.microsoftonline.com/common/oauth2/v2.0/authorize" +
@@ -61,7 +66,8 @@ fun launchOneDriveOAuth(context: Context, credentialStore: CredentialStore, clie
         "&redirect_uri=syncflow%3A%2F%2Foauth%2Fonedrive" +
         "&scope=$scopes" +
         "&code_challenge=$challenge" +
-        "&code_challenge_method=S256"
+        "&code_challenge_method=S256" +
+        "&state=$state"
     openCustomTab(context, url)
 }