Remove CSS Rounded Edges on Proposal Creation Pages on background

[bosko-m]

Description

Remove all CSS border-radius (rounded edges) styling from the proposal creation views background in both:

• Proposal Discussion Forum
• Budget Proposals

This change ensures consistency with the design system or improves visual alignment across the application.

Acceptance Criteria

• All rounded edges are removed from background in the Proposal Discussion Forum proposal creation flow.
• All rounded edges are removed from background in the Budget Proposals creation flow.

[bosko-m]

Validated on dev.

[kneerose]

@MihajlovicMilos1995
Same CSS rounded edges are available on create a governance action

CC: @bosko-m

[bosko-m]

Good catch @kneerose , although not a showstopper for the release. Created separate ticket for it: #3705