feat: add MilestoneBreakdown component for proposal submission

[Xalavina]

Overview

This PR introduces the MilestoneBreakdown component to support the structured proposal submission interface. It enables freelancers to outline deliverables, timelines, and partial payments as an optional section within their proposals, ensuring clarity in expectations and facilitating efficient client evaluation.

Related Issue

Changes

📦 Proposal Submission Interface

• [ADD] components/proposal/MilestoneBreakdown.tsx
  • Implements a card-based layout for outlining milestone deliverables.
  • Supports optional inclusion; proposals without milestones remain valid.
  • Includes inline validation messages for timeline and payment fields.
  • Integrates with the parent proposal form to handle structured milestone data.

Verification Results

Manual Verification:
✅ Milestone breakdown renders correctly as a card-based layout
✅ Form remains valid when milestone section is left empty (optional)
✅ Inline validation messages appear for invalid date/numeric inputs
✅ Component adapts responsively to desktop, tablet, and mobile views

Acceptance Criteria	Status
Optional milestone breakdown	✅ Implemented with card-based layout
Responsive design	✅ Adapts seamlessly across devices
Inline validation messages	✅ Provided for structured fields
Proposal status visibility	✅ Handled by parent component (not in scope of this file)

Closes #212

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