[codex] Add paginated conversation detail API for mobile

[ijry]

Summary

This is a conflict-free replacement for #283, rebased on the current xintaofei/codeg:main.

It adds a mobile-oriented paginated conversation detail API while leaving the existing full-detail API intact:

• adds DbConversationDetailPage for summaries plus a window of turns
• adds get_folder_conversation_paginated for both Tauri commands and the Axum web API
• exposes the new API through the frontend API/Tauri wrappers and TypeScript types
• adds unit coverage for pagination boundaries

Rebase notes

The old #283 branch conflicted in src/components/message/message-list-view.tsx. That file was only carrying a formatting/layout conflict from the old branch, not the paginated API behavior, so this replacement keeps the current main version of that component and retains only the API/model/client changes.

The compile issue after replaying the old commit was caused by the new DbConversationDetailPage model not being exported through src-tauri/src/models/mod.rs, while the command and web handler modules rely on crate::models::*. This PR exports the type at the model boundary instead of adding one-off imports in each consumer.

Compatibility

The existing get_folder_conversation response shape and route remain unchanged. Mobile clients can opt into get_folder_conversation_paginated with optional beforeTurnIndex and pageSize parameters. The first call defaults to the latest 30 turns, then older pages can be requested with next_before_turn_index while has_more_history is true.

Validation

• cargo test --no-default-features --lib paginate_turns
• cargo check --no-default-features --bin codeg-server
• pnpm exec eslint src/lib/api.ts src/lib/tauri.ts src/lib/types.ts
• pnpm exec tsc --noEmit
• git diff --check