feat: add duplicate account import prevention

- Add duplicate check in ImportAccountView before account import
- Add duplicate check in ImportDerivedAccountView for derived accounts
- Add safety check in saveNewIdentity function to prevent duplicate saves
- Implement user-friendly warning messages for duplicate attempts
- Add comprehensive error handling to catch duplicate errors from saveNewIdentity
- Create Playwright tests to verify duplicate prevention functionality
- Add documentation for duplicate prevention implementation

The system now prevents users from importing the same account multiple times
by checking for existing DIDs both before import (pre-check) and during
save (post-check). Users receive clear warning messages instead of
technical errors when attempting to import duplicate accounts.

Files modified:
- src/views/ImportAccountView.vue: Add duplicate check and error handling
- src/views/ImportDerivedAccountView.vue: Add duplicate check for derived accounts
- src/libs/util.ts: Add duplicate prevention in saveNewIdentity
- test-playwright/duplicate-import-test.spec.ts: Add comprehensive tests
- doc/duplicate-account-import-implementation.md: Add implementation docs

Resolves: Prevent duplicate account imports in IdentitySwitcherView

src/libs/util.ts

@@ -626,6 +626,18 @@ export async function saveNewIdentity(
     // add to the new sql db
     const platformService = await getPlatformService();
 
+    // Check if account already exists before attempting to save
+    const existingAccount = await platformService.dbQuery(
+      "SELECT did FROM accounts WHERE did = ?",
+      [identity.did],
+    );
+
+    if (existingAccount?.values?.length) {
+      throw new Error(
+        `Account with DID ${identity.did} already exists. Cannot import duplicate account.`,
+      );
+    }
+
     const secrets = await platformService.dbQuery(
       `SELECT secretBase64 FROM secret`,
     );
@@ -660,10 +672,8 @@ export async function saveNewIdentity(
 
     await platformService.insertNewDidIntoSettings(identity.did);
   } catch (error) {
-    logger.error("Failed to update default settings:", error);
-    throw new Error(
-      "Failed to set default settings. Please try again or restart the app.",
-    );
+    logger.error("Failed to save new identity:", error);
+    throw error;
   }
 }