fix: fixed shape of search filters dropdown

[sphurthy]

Description

Related PRS (if any):

None

Main changes explained:

• Added rectangularDropdown style with border-radius: 0.
• Applied it to Branches, Themes, Categories dropdowns.
• Applied it to the date (mm/dd/yyyy) input.
• UI-only change; no functionality updated.

How to test:

1. check into current branch
2. do npm install and ... to run this PR locally
3. Clear site data/cache
4. Open /communityportal -> All Events.
5. Check Search Filters: all 4 fields now have sharp corners.
6. Try selecting date/dropdowns to confirm they still work.

Screenshots or videos of changes:

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud