Public trips - Build offer to host - step 1

[nabramow]

What's NOT in this PR:

Anything on the messages screen will be in a separate PR. Will not work in Vercel as there is backend, must run locally and pull branch. Everything is behind a FF.

This PR:

• Create "offer to host" dialog that creates the offer
• Shows if you already offered with a deep link to the convo (includes backend)

Testing

Explain how you tested this PR and give clear steps so the reviewer can replicate.

• cd app/web && cp .env.localdev .env.development && yarn start
• Run backend locally docker compose up --build
• Login with local aapeli account and create a public trip in a community
• Open private window and log in with username kiritimati dummy account, pw: kirikiri and create a public trip
• Offer to host from one account to the other and check how it looks on in the public trips tab of a community

Backend checklist

• Added tests for any new code or added a regression test if fixing a bug
• Run the backend locally and it works
• Added migrations if there are any database changes, rebased onto develop if necessary for linear migration history

Web frontend checklist

• There are no console warnings when running the app
• Added tests where relevant
• Clicked around my changes running locally and it works
• Checked Desktop, Mobile and Tablet screen sizes

For maintainers

• Maintainers can push commits to my branch
• Maintainers can merge this PR for me

[CouchersBot]

Mobile

Install the Dev Tool (iOS via TestFlight, iOS Simulator, or Android .apk) here.

Scan the QR with your phone camera, or tap Open in Dev Tool on the device, to open this branch in the installed Dev Tool dev client.

iOS	Android
Open in Dev Tool	Open in Dev Tool

Deep links

iOS

couchers-devtool://expo-development-client/?url=https%3A%2F%2F0a940bb5--ota.preview.couchershq.org%2Fios%2Fmanifest

Android

couchers-devtool://expo-development-client/?url=https%3A%2F%2F0a940bb5--ota.preview.couchershq.org%2Fandroid%2Fmanifest

Backend

Schema	Schema diff	Sample emails

Other

Backend coverage	Web coverage

[github-actions]

📝 Release Notes

This PR should be included in the release notes.

Suggested release note:

Added the ability to offer to host travelers directly from public trips and quickly return to an existing offer conversation

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: include
Reasoning: This PR adds a new user-facing feature in public trips: hosts can now send an offer to host directly from a public trip, and users can see when they have already offered and jump to that conversation. Although it is behind a feature flag and labeled step 1, it introduces meaningful visible functionality for end users.