refactor: Remove defunct $needsActiveIdentitySelection method and fix activeDid consistency

## Changes Made

### Code Cleanup
- Remove unused $needsActiveIdentitySelection() method (36 lines)
- Remove method signature from IPlatformServiceMixin interface
- Remove method signature from Vue module declaration

### Database Consistency Fix
- Change activeDid clearing from empty string ('') to NULL for consistency
- Ensures proper foreign key constraint compatibility
- Maintains API compatibility by still returning empty string to components

## Impact
- Reduces codebase complexity by removing unused functionality
- Improves database integrity with consistent NULL usage
- No breaking changes to component APIs
- Migration and auto-selection now handle all identity management

## Files Changed
- src/utils/PlatformServiceMixin.ts: -42 lines, +1 line

src/utils/PlatformServiceMixin.ts

@@ -236,45 +236,6 @@ export const PlatformServiceMixin = {
       }
     },
 
-    /**
-     * Check if active identity needs user intervention
-     * Returns true if active_identity table is empty but accounts exist
-     * This allows components to show appropriate UI for user selection
-     */
-    async $needsActiveIdentitySelection(): Promise<boolean> {
-      try {
-        // Check if active_identity table has a valid activeDid
-        const activeIdentity = await this.$dbQuery(
-          "SELECT activeDid FROM active_identity WHERE id = 1",
-        );
-
-        const currentActiveDid = activeIdentity?.values?.[0]?.[0] as string;
-
-        // If we have an activeDid, validate it exists in accounts
-        if (currentActiveDid) {
-          const accountExists = await this.$dbQuery(
-            "SELECT did FROM accounts WHERE did = ?",
-            [currentActiveDid],
-          );
-          return !accountExists?.values?.length;
-        }
-
-        // If no activeDid, check if there are any accounts available
-        const availableAccounts = await this.$dbQuery(
-          "SELECT COUNT(*) FROM accounts",
-        );
-        const accountCount = availableAccounts?.values?.[0]?.[0] as number;
-
-        return accountCount > 0;
-      } catch (error) {
-        logger.error(
-          "[PlatformServiceMixin] Error checking if active identity selection needed:",
-          error,
-        );
-        return false;
-      }
-    },
-
     /**
      * Get available account DIDs for user selection
      * Returns array of DIDs that can be set as active identity
@@ -722,7 +683,7 @@ export const PlatformServiceMixin = {
                 "[PlatformServiceMixin] Active identity not found in accounts, clearing",
               );
               await this.$dbExec(
-                "UPDATE active_identity SET activeDid = '', lastUpdated = datetime('now') WHERE id = 1",
+                "UPDATE active_identity SET activeDid = NULL, lastUpdated = datetime('now') WHERE id = 1",
               );
               return { activeDid: "" };
             }
@@ -1975,7 +1936,6 @@ export interface IPlatformServiceMixin {
   ): Promise<Settings>;
   $getActiveIdentity(): Promise<{ activeDid: string }>;
   $withTransaction<T>(callback: () => Promise<T>): Promise<T>;
-  $needsActiveIdentitySelection(): Promise<boolean>;
   $getAvailableAccountDids(): Promise<string[]>;
   isCapacitor: boolean;
   isWeb: boolean;
@@ -2100,7 +2060,6 @@ declare module "@vue/runtime-core" {
     ): Promise<Settings>;
     $getActiveIdentity(): Promise<{ activeDid: string }>;
     $withTransaction<T>(fn: () => Promise<T>): Promise<T>;
-    $needsActiveIdentitySelection(): Promise<boolean>;
     $getAvailableAccountDids(): Promise<string[]>;
 
     // Specialized shortcuts - contacts cached, settings fresh