[G-ELM]

Summary

Adds four documentation pages.

• contracts/docs/contracts-proposals-storage.md
  • Covers the proposals storage keys and the Proposal struct.
  • Covers the full ProposalStatus enum with a transition table.
  • Covers vote tracking and the instance-storage and TTL behaviour (the contract never extends TTL).
  • Explains how TreasuryClient is generated and how the contract address is resolved and invoked.
  • Notes that treasury_interface_client.rs is an empty, uncompiled placeholder.
  • Notes that ProposalStatus::Approved is never set.
• apps/web/docs/concepts-app-routing.md
  • Gives the App Router tree with a one-line purpose for each route under app/app/.
  • Describes the conversation view: hooks and libraries, data flow, message list, input bar, and the safety-number UI.
  • Written against the structure after the route consolidation.
  • Flags unused leftovers (providers.tsx, routes/conversations.js).
• docs/user-guide/getting-started.md
  • Plain-language guide covering Freighter install, wallet connection, sign-in, starting a conversation, sending a file, and sending tokens.
  • Explains that sign-in signs a one-time challenge and does not spend funds or grant spending permission.
  • Avoids internal terminology.
• contracts/docs/concepts-cross-contract-interfaces.md
  • Documents #[contractclient] and every token and treasury interface.
  • Explains the two withdrawal paths: the treasury's own multisig, and proposals::execute_withdraw.
  • Describes how they relate: parallel, not layered. Only the admin-gated withdraw moves funds, and it needs treasury admin authorization.

Closes #470
Closes #463
Closes #643
Closes #631

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