feat: Add end field: backend + frontend; validate end >= start; update swagger

[jnakaigen]

Description

Adds an end field to tasks (backend models, request bodies, Taskwarrior helpers).

Validates on server that end is greater than or equal to start.

Wires end through the frontend Add/Edit task UI and API payloads.

Updates Swagger docs to document the new field.

Fixes: #188

Checklist

• Ran npx prettier --write . (for formatting)
• Ran gofmt -w . (for Go backend)
• Ran npm test (for JS/TS testing)
• Added unit tests, if applicable
• Verified all tests pass
• Updated documentation, if needed

Additional Notes

Backend tests that invoke external binaries will fail on Windows unless run inside Docker/WSL or the CI environment. I validated end-to-end task add in Docker after updating .env to use the syncserver service name.

[github-actions]

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[jnakaigen]

Thanks for the guidance!
I’ve gone through the Files changed tab and added comments where I thought clarification was needed (e.g., function naming choices and validation logic).
I also explained why I added the end field and the validation check (end >= start), and noted a couple of areas that might need further discussion in future updates.
Please let me know if there’s anything else I should expand on in my self-review.

[its-me-abhishek]

@jnakaigen this got fixed by #293 apparently, closing for now!