feat: hide time in proposal start date filter

[abdimo101]

Description

This PR updates the proposal start date filter to hide the time portion, displaying only the date (example 2025-10-08) instead of including the time.

Motivation

• https://jira.ess.eu/browse/SWAP-5041

Fixes:

Please provide a list of the fixes implemented in this PR

• Items added

Changes:

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• changes made

Tests included

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• Passing? (Merge will not be approved unless this is checked)

Documentation

• swagger documentation updated [required]
• official documentation updated [nice-to-have]

official documentation info

If you have updated the official documentation, please provide PR # and URL of the pages where the updates are included

Backend version

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Summary by Sourcery

Configure proposal start date filter to display only the date without time by providing custom Angular Material date formats

New Features:

• Hide the time portion in proposal start date filter and display only the date

Enhancements:

• Define PROPOSAL_DATE_FORMATS constant and register it via MAT_DATE_FORMATS in ProposalsModule

[sourcery-ai]

Hey there - I've reviewed your changes and they look great!

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