feat: add draft proposal state and workflow

[jaydenkalu]

Closes PrincessnJoy#375

Changes

• Updated frontend/src/types.ts — added 'Draft' to ProposalState union and added DraftProposal interface
• Added frontend/src/drafts.ts — localStorage-based CRUD for draft proposals (saveDraft, getDrafts, getDraft, deleteDraft)
• Added frontend/src/components/DraftProposalList.tsx — UI component listing saved drafts with Edit, Delete, and Publish actions
• Updated frontend/src/components/NewProposalForm.tsx — added initialDraft prop for pre-population and a Save as Draft button

Design Notes

Drafts are stored client-side in localStorage (off-chain). The on-chain Soroban contract is unchanged. A draft must be explicitly published (submitted on-chain via create_proposal) to become an Active proposal — enforcing the requirement that voting cannot start until the proposal is finalized.

Acceptance Criteria

• Draft proposal state added (ProposalState includes 'Draft')
• Drafts can be edited before publishing; votes only start after on-chain submission
• UI for saving and publishing drafts (DraftProposalList + Save as Draft button)