feat: add calendar export for project deadlines and milestones

[Okorie2000-code]

• Add RFC 5545-compliant ICS generation utility (generateICS, downloadICS)
• Add 'Add to Calendar' button on project detail page
• Export milestone due dates as calendar events with UID, title, date, description
• Skip milestones with missing dates gracefully
• Add unit tests covering ICS structure, fields, CRLF, deduplication, and download

What this PR does

• Summary of the implemented changes related to issue(s) DevTooling: Implement Monorepo with Turborepo for Shared Packages #240 and DevTooling: Implement Real-Time Error Monitoring with Sentry and Custom Dashboards #242
• Any breaking changes introduced (if any)
• Any migrations or data changes (if applicable)

Changes

• List of notable files and modules touched (monorepo wiring, Sentry integration, dashboards, etc.)
• Notes about the Turborepo configuration, shared packages, and ESLint/TS config

Why

• Rationale for the changes and how they solve the Acceptance Criteria

How to test

• Steps to reproduce locally, including commands to run build/test
• Any environment variables needed (e.g., SENTRY_DSN)
• How to verify the dashboards/alerts configuration (stubs or real)

Notes

• Any caveats, risks, or follow-up tasks
• If you want to close issues automatically, include:
  • Closes DevTooling: Implement Monorepo with Turborepo for Shared Packages #240
  • Closes DevTooling: Implement Real-Time Error Monitoring with Sentry and Custom Dashboards #242

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