refactor(permissions): centralize permission checks

[compgeniuses]

Description

• Replace hardcoded role permissions in ProtectedRoute with reusable hasPermission utility
• Update nav items and routes to use standardized PERMISSIONS enum values
• Fix Accounts page permission check to use correct ACCOUNTS_MANAGE permission
• Add type safety and explanatory comments to auth and route files
  Fixes #(issue)

Type of change

• [-] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor (no functional changes)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

• [-] Unit tests
• [-] Integration tests
• [-] End-to-end tests
• [-] Manual testing

Test Configuration:

• Node.js version:
• PostgreSQL version:
• OS:

Checklist:

• My code follows the project's code style
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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Additional context:

Add any other context about the pull request here.