feat(treasury): approval/rejection voting UI for multisig proposals (#132)

[Jaydams]

Closes #132 — builds the full approval/rejection voting flow for multisig
treasury proposals, from the database layer through the API to the UI.

Backend

• Schema (apps/backend/src/db/schema.ts): extended treasury_proposals
  with recipient, amount, token, and threshold columns so proposal
  cards can render useful detail; added proposal_vote_type enum and a new
  proposal_votes table (unique on (treasury_proposal_id, user_id)) that
  records each signer's vote and optional Freighter signature; added Drizzle
  relations for both tables.

• Migration (drizzle/0008_treasury_multisig_voting.sql): creates the
  treasury_proposals table (first migration for it), adds the four new
  columns, and creates proposal_votes with FK constraints and a unique
  partial index to prevent double-voting at the DB level.

• Treasury router (src/routes/treasury.ts):

  • POST /treasury/propose — now persists proposals to the DB (was a stub
    returning TTL only); accepts optional conversationId and threshold.
  • GET /treasury/proposals?conversationId= — returns all proposals,
    optionally filtered by conversation; annotates each row with hasVoted
    and myVote for the authenticated user via a single IN-query on
    proposal_votes.
  • POST /treasury/proposals/:id/approve and .../reject — validates the
    proposal is still active, inserts a proposal_votes row, and returns
    409 on a unique-constraint violation (duplicate vote) or on an inactive
    proposal.

Frontend

• ProposalCard component (src/components/treasury/ProposalCard.tsx):
  shows proposal ID, truncated recipient address, amount + token, a colour-coded
  status badge (Pending / Approved / Rejected / Executed / Expired), an
  approval progress bar (approvalsCount / threshold), and Approve/Reject
  buttons. Buttons are disabled when hasVoted is true or status !== "active".
  On click, Freighter signs "approve:<proposalId>" / "reject:<proposalId>"
  before submitting to the API; a toast is shown if signing is cancelled or the
  request fails.

• Treasury page (src/app/app/treasury/page.tsx):

  • Fetches GET /treasury/proposals on mount (and after a new proposal is
    created via the modal).
  • Opens a Socket.IO connection and listens for treasury_proposal_updated
    events, patching status, approvalsCount, and rejectionsCount in local
    state without a full re-fetch.
  • Renders a responsive 1–3 column ProposalCard grid with skeleton loading
    and an empty state.
  • "Pending Transactions" summary card now reflects the live count of active
    proposals.

Test plan

• POST /treasury/propose with a valid body → 201, proposal appears in
  GET /treasury/proposals
• GET /treasury/proposals returns hasVoted: false / myVote: null
  before voting
• Click Approve on a card → Freighter prompt appears → on confirm,
  button shows "Approved ✓", both buttons disable
• Click Reject → same flow with "Rejected ✗"
• Voting a second time → toast shows "Already voted on this proposal"
• Approve/Reject on a non-active proposal → 409 with descriptive error
• Cancel Freighter prompt → toast shows "Freighter signing was cancelled or
  failed", no API call made
• Backend emits treasury_proposal_updated → treasury page updates counts
  in real time without manual refresh
• "Pending Transactions" card count matches number of active proposals

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