Core: Implement Token Allowances Management Dashboard

[Smartdevs17]

Description\nImplement token allowance management dashboard for users to view, approve, and revoke token allowances across contracts. Required for ERC-20 security.\n\nCurrent Limitation: No allowance visibility.\nExpected Outcome: Dashboard showing all allowances with approval/revocation and risk indicators.\n## Acceptance Criteria\n- [ ] Allowance fetch from chain\n- [ ] Approval/revocation interface\n- [ ] Risk indicators for unlimited allowances\n- [ ] Batch revocation\n- [ ] Approval change notifications\n- [ ] Historical approvals log\n- [ ] Gas estimation for changes\n- [ ] Recommended allowance amounts\n## Technical Scope\n- Files: frontend/dashboard/allowances, backend allowances API\n- APIs: /api/v1/allowances\n- Edge Cases: Revoked allowances, pending transactions\n## Complexity Estimate\n200 points - Chain queries, approval/decreaseAllowance calls.

[Dev-sandy1]

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