Fix Navbar, add PR checks, UI/UX improvements

[antonio-rodriguez-tam]

Fix Navbar, add PR checks, UI/UX improvements

Description:

• Include missing navbar template file (staff/superuser only).
• Add GitHub Actions CI: unit tests (Python 3.10–3.12 with coverage), flake8 lint, and package build/wheel verification.
• Improve compose page: template search, recipient filter layout (filters row + actions row), recipient preview modal with fixed-height scrollable table, pagination with selectable page size, and client-side search.
• Add resend-compose page: review and re-send a previous campaign with editable subject/body, email preview, recipient preview, and inline validation.
• Sort notification tables by status priority (failed first) then by date descending.
• Fix flake8 lint errors across resolvers, validators, views, forms, admin, utils, and tasks.
• Add 21 unit tests for _build_values, _send_with_retry, validate_campaign, _primary_resource, and resolver exception handling.
• Add management command seed_notification_senders to bootstrap SenderConfig from EMAIL_SENDER, EMAIL_TICKET_SYSTEM_ADDRESS, EMAIL_DIRECTOR_EMAIL_ADDRESS.
• Restrict navbar visibility to staff/superuser users.

Security Checklist

• Input Validation: All user inputs are properly validated and sanitized to prevent injection attacks.
• [ X] Authentication & Authorization: The PR does not introduce unauthorized access, public endpoints or privilege escalation risks.
• [ X] Secrets Management: No hardcoded secrets, API keys, or sensitive data are present.
• [ X] Data Protection: Sensitive data is encrypted both at rest and in transit where applicable.
• [ X] Logging & Monitoring: New features include proper logging and monitoring where necessary, without logging sensitive information.
• [ X] Security Headers: If applicable, relevant security headers (CSP, HSTS, X-Frame-Options) are correctly configured.
• [ X] Error Handling: Errors do not expose sensitive system details.
• [X ] Third-Party Services: Any new third-party integrations follow security best practices (e.g. authentication, encryption, logging).
• [ X] Code Review: The PR has been reviewed for potential security flaws.

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Additional Notes

Checklist

• I have performed a self-review of my code
• I have tested the changes locally
• I have run security tests where applicable
• This PR follows the coding and security guidelines of the project

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Reviewer(s):
@mention reviewers