feat: add multisig transaction builder ui

[iammrjude]

Description

Implements an interactive multi-signature transaction builder for teams, with proposal creation, visual signature progress, signature verification, JSON sharing, common templates, and notification requests for pending signers.

Closes #343

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Changes Made

• Added starforge multisig wizard interactive proposal flow
• Added visual signature progress tracking for proposal status/view flows
• Added proposal metadata, transaction XDR support, event history, and backward-compatible JSON fields
• Added signature payload generation, duplicate checks, validation reports, and multisig verify
• Added multisig notify to queue signature request notifications for pending signers
• Added common templates for escrow, company treasury, DAO, vault, and payment workflows
• Updated command reference docs and added focused tests

Testing

cargo check
cargo test --test multisig_builder_ui -- --nocapture
cargo test multisig_builder --lib -- --nocapture

Test Coverage

• Happy path: template proposal creation, progress tracking, valid signature collection, readiness detection
• Edge cases: duplicate signatures rejected, pending signer tracking
• Error handling: invalid signatures reported and excluded from readiness

Breaking Changes

None. Existing proposal JSON remains backward-compatible through serde defaults.

Documentation

Updated docs/COMMAND_REFERENCE.md with the new multisig commands and examples.

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