Concepts docs: App routing & conversation UI architecture

[codebestia]

Write apps/web/docs/concepts-app-routing.md explaining the Next.js App Router structure under app/, the authenticated app/app/ shell and its child routes (conversations, devices, messages, profile, proposals, search, treasury), and how the conversation view composes its message list, input, and safety-number UI.

Acceptance criteria:

• The route tree is documented with a one-line purpose for each top-level route under app/app/
• The conversation route's composition (which components/hooks it pulls together) is documented at a level that orients a new contributor
• Written against the routing structure as it exists after the route-consolidation work

[Toyinoje]

@Toyinoje has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello please This issue aligns well with my experience. I can work on it and submit a polished PR for review

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Toyinoje to this issue.

[drips-wave]

Congratulations, @psalmuel3! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @psalmuel3: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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