fix(frontend): allow creating a project without an end date

[nourshoreibah]

Summary

• Leaving End Date blank on create/edit no longer fails with "Please select a date AFTER the start date".
• An end date is still rejected when it is filled in and precedes the start date.
• Matches the API, which already treats end_date as optional.

Test plan

• Open Add New Project, fill required fields, leave End Date empty, do not check "still in progress", and Save — it should succeed.
• Create a project with an end date before the start date — it should still show the after-start-date error.
• Create a project with a valid end date, and one with "still in progress" checked — both should still work.

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