Add Governance module: proposals, weighted voting, feature-gated UI

[Six2world]

closes #337

Summary

Adds a complete, feature-flagged Governance module enabling communities to propose, discuss, and vote on decisions.

Changes

• Types & API contract: Proposal/Vote types with full lifecycle (draft/active/closed/resolved), plus a 15-endpoint API specification (docs/governance-contract.md)
• Mock backend: In-memory governance engine with weighted voting (tier x role, up to 9x for pro admin), vote aggregation, community-scoped storage, and localStorage persistence for demos
• Live backend client: 10 methods implementing the governance API contract, with graceful degradation on 503 (returns safe empty defaults instead of crashing)
• Frontend routes: proposal list (with status filters), proposal detail (voting UI + results), and admin-only create proposal form
• Query keys: added for list/detail/memberVote/votes

Key characteristics

• Feature-gated behind NEXT_PUBLIC_FEATURE_GOVERNANCE, hidden by default
• Role-based access: members vote, admins create/manage proposals
• Uses existing SIWE wallet auth for all governance actions
• Fully functional in mock mode without a backend; ready for guildpass-core backend implementation

Testing

Fully mock-complete and manually verified across proposal lifecycle (draft → active → closed → resolved). Type-safe with zero TypeScript errors.

[Lakes41]

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