refactor: use concrete SdkError type in error variants; drop dyn Error

- IdentityUpdateTransitionError.source_error: Box<dyn Error> → Box<SdkError>
  (ProtocolError converts via SdkError::Protocol(e))
- InternalSendError: struct variant → unit variant (SendError carries no
  diagnostic information worth preserving in the chain)
- CLAUDE.md rule 8: document source field type conventions — Box<SdkError>
  for SDK-originated errors, concrete domain type for non-SDK errors,
  omit #[source] when upstream carries no diagnostic value

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

Author: lklimek

CLAUDE.md

@@ -73,7 +73,7 @@ User-facing error messages (shown in `MessageBanner` via `Display`) must follow
 5. **i18n-ready**: Write messages as simple, complete sentences without interpolation tricks. Avoid concatenating fragments, positional assumptions, or grammar that breaks in other languages. Messages should be straightforward to extract into [Fluent](https://projectfluent.org/) `.ftl` files later — one message ID per string, placeholders only for dynamic values (`{ $seconds }`, `{ $name }`), no logic in the text itself.
 6. **Reference implementation**: `sdk_error_user_message()` in `src/backend_task/error.rs` demonstrates the pattern for SDK errors. New `TaskError` variants should follow the same style.
 7. **Base58 IDs are allowed in messages**: Contract IDs, identity IDs, document IDs, and similar Base58-encoded identifiers may appear in user-facing messages when they help the user identify which object is involved (e.g., *"This key conflicts with an existing key bound to contract `Abc123…`."*). They are not jargon — they are opaque-but-copyable handles the user can look up.
-8. **Prefer granular `TaskError` variants over `Generic`**: When mapping errors to add context, add a dedicated `TaskError` variant with a `#[source]` field rather than converting to `TaskError::Generic(format!(...))`. Granular variants preserve the error chain, enable structural matching by callers, and make `Display` / `Debug` separation explicit. `TaskError::Generic` is a last resort for one-off strings with no upstream error to preserve.
+8. **Prefer granular `TaskError` variants over `Generic`**: When mapping errors to add context, add a dedicated `TaskError` variant with a `#[source]` field rather than converting to `TaskError::Generic(format!(...))`. Granular variants preserve the error chain, enable structural matching by callers, and make `Display` / `Debug` separation explicit. `TaskError::Generic` is a last resort for one-off strings with no upstream error to preserve. For `#[source]` fields in SDK-originated error variants, use `Box<SdkError>` — convert upstream types (e.g. `ProtocolError`) via `SdkError::Protocol(e)`. Use the concrete domain type directly for non-SDK errors (e.g. `rusqlite::Error`). Omit `#[source]` entirely when the upstream error carries no useful diagnostic information (e.g. a channel `SendError`).
 
 ## Architecture Overview
 

src/backend_task/error.rs

@@ -110,15 +110,12 @@ pub enum TaskError {
     #[error("Could not build the key update transaction. Please retry.")]
     IdentityUpdateTransitionError {
         #[source]
-        source: Box<dyn std::error::Error + Send + Sync>,
+        source_error: Box<SdkError>,
     },
 
     /// Failed to send a result back to the UI — the receiver was dropped.
     #[error("Internal update failed. Please retry the operation.")]
-    InternalSendError {
-        #[source]
-        source: Box<dyn std::error::Error + Send + Sync>,
-    },
+    InternalSendError,
 
     /// Unclassified SDK error — the operation failed for an unrecognised reason.
     /// Display is implemented manually via [`sdk_error_user_message`] to inspect

src/backend_task/identity/add_key_to_identity.rs

@@ -6,6 +6,7 @@ use crate::model::fee_estimation::PlatformFeeEstimator;
 use crate::model::qualified_identity::PrivateKeyTarget::PrivateKeyOnMainIdentity;
 use crate::model::qualified_identity::QualifiedIdentity;
 use crate::model::qualified_identity::qualified_identity_public_key::QualifiedIdentityPublicKey;
+use dash_sdk::Error as SdkError;
 use dash_sdk::Sdk;
 use dash_sdk::dpp::identity::accessors::{IdentityGettersV0, IdentitySettersV0};
 use dash_sdk::dpp::identity::identity_public_key::accessors::v0::{
@@ -64,7 +65,7 @@ impl AppContext {
             None,
         )
         .map_err(|e| TaskError::IdentityUpdateTransitionError {
-            source: Box::new(e),
+            source_error: Box::new(SdkError::Protocol(e)),
         })?;
 
         let result = state_transition.broadcast_and_wait(sdk, None).await?;

src/backend_task/identity/refresh_identity.rs

@@ -55,9 +55,7 @@ impl AppContext {
         sender
             .send(TaskResult::Refresh)
             .await
-            .map_err(|e| TaskError::InternalSendError {
-                source: Box::new(e),
-            })?;
+            .map_err(|_| TaskError::InternalSendError)?;
 
         Ok(BackendTaskSuccessResult::RefreshedIdentity(
             qualified_identity,