feat(groups): add rate multipliers management modal

Add a dedicated modal in group management for viewing, adding, editing,
and deleting per-user rate multipliers within a group.

Backend:
- GET /admin/groups/:id/rate-multipliers - list entries with user details
- PUT /admin/groups/:id/rate-multipliers - batch sync (full replace)
- DELETE /admin/groups/:id/rate-multipliers - clear all entries
- Repository: GetByGroupID, SyncGroupRateMultipliers methods on
  user_group_rate_multipliers table (same table as user-side rates)

Frontend:
- New GroupRateMultipliersModal component with:
  - User search and add with email autocomplete
  - Editable rate column with local edit mode (cancel/save)
  - Batch adjust: multiply all rates by a factor
  - Clear all (local operation, requires save to persist)
  - Pagination (10/20/50 per page)
  - Platform icon with brand colors in group info bar
  - Unsaved changes indicator with revert option
- Unit tests for all three backend endpoints

frontend/src/i18n/locales/en.ts

@@ -1372,7 +1372,11 @@ export default {
         accounts: 'Accounts',
         status: 'Status',
         actions: 'Actions',
-        billingType: 'Billing Type'
+        billingType: 'Billing Type',
+        userName: 'Username',
+        userEmail: 'Email',
+        userNotes: 'Notes',
+        userStatus: 'Status'
       },
       rateAndAccounts: '{rate}x rate · {count} accounts',
       accountsCount: '{count} accounts',
@@ -1411,6 +1415,26 @@ export default {
       failedToUpdate: 'Failed to update group',
       failedToDelete: 'Failed to delete group',
       nameRequired: 'Please enter group name',
+      rateMultipliers: 'Rate Multipliers',
+      rateMultipliersTitle: 'Group Rate Multipliers',
+      addUserRate: 'Add User Rate Multiplier',
+      searchUserPlaceholder: 'Search user email...',
+      noRateMultipliers: 'No user rate multipliers configured',
+      rateUpdated: 'Rate multiplier updated',
+      rateDeleted: 'Rate multiplier removed',
+      rateAdded: 'Rate multiplier added',
+      clearAll: 'Clear All',
+      confirmClearAll: 'Are you sure you want to clear all rate multiplier settings for this group? This cannot be undone.',
+      rateCleared: 'All rate multipliers cleared',
+      batchAdjust: 'Batch Adjust Rates',
+      multiplierFactor: 'Factor',
+      applyMultiplier: 'Apply',
+      rateAdjusted: 'Rates adjusted successfully',
+      rateSaved: 'Rate multipliers saved',
+      finalRate: 'Final Rate',
+      unsavedChanges: 'Unsaved changes',
+      revertChanges: 'Revert',
+      userInfo: 'User Info',
       platforms: {
         all: 'All Platforms',
         anthropic: 'Anthropic',