fix: remove incorrect "globally unique" claim from error messages

Only unique key types (ECDSA_SECP256K1, BLS12_381) require platform-wide
uniqueness. Non-unique types (ECDSA_HASH160, BIP13_SCRIPT_HASH,
EDDSA_25519_HASH160) can be shared across identities. Simplified messages
to avoid misleading users about key uniqueness rules.

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

Author: lklimek

docs/ai-design/2026-03-11-duplicate-key-error/manual-test-scenarios.md

@@ -38,7 +38,7 @@ a key whose public key data already exists on the identity.
 ### Expected Result
 
 - An **error banner** appears at the top of the screen with the message:
-  > This public key is already registered on the platform. Each key must be globally unique — try a different key.
+  > This public key is already registered on the platform. Try a different key.
 - The message is plain text -- no base64, no CBOR encoding, no raw error dump.
 - The banner type is **Error** (red/error styling).
 - The app does **not** crash or freeze.
@@ -74,7 +74,7 @@ error display path regardless of trigger.
 ### Expected Result
 
 - An **error banner** appears with the message:
-  > This key hash is already registered on the platform. Each key must be globally unique — try a different key.
+  > This key hash is already registered on the platform. Try a different key.
 - The message is plain text -- no encoded data.
 - The banner type is **Error**.
 - The app does **not** crash.

src/backend_task/error.rs

@@ -62,15 +62,11 @@ pub enum TaskError {
     MustRetry(String),
 
     /// Duplicate identity public key — the key data already exists on the platform.
-    #[error(
-        "This public key is already registered on the platform. Each key must be globally unique — try a different key."
-    )]
+    #[error("This public key is already registered on the platform. Try a different key.")]
     DuplicateIdentityPublicKey,
 
     /// Duplicate identity public key ID — the key hash is already taken platform-wide.
-    #[error(
-        "This key hash is already registered on the platform. Each key must be globally unique — try a different key."
-    )]
+    #[error("This key hash is already registered on the platform. Try a different key.")]
     DuplicateIdentityPublicKeyId,
 
     /// Identity public key conflicts with an existing key's unique contract bounds.

src/backend_task/identity/add_key_to_identity.rs

@@ -179,7 +179,7 @@ mod tests {
         let msg = broadcast_error_message(&sdk_err);
         assert_eq!(
             msg,
-            "This public key is already registered on the platform. Each key must be globally unique — try a different key."
+            "This public key is already registered on the platform. Try a different key."
         );
     }
 
@@ -190,7 +190,7 @@ mod tests {
         let msg = broadcast_error_message(&sdk_err);
         assert_eq!(
             msg,
-            "This key hash is already registered on the platform. Each key must be globally unique — try a different key."
+            "This key hash is already registered on the platform. Try a different key."
         );
     }
 
@@ -232,7 +232,7 @@ mod tests {
         let msg = broadcast_error_message(&sdk_err);
         assert_eq!(
             msg,
-            "This public key is already registered on the platform. Each key must be globally unique — try a different key."
+            "This public key is already registered on the platform. Try a different key."
         );
     }