refactor: trim unused client surface

[KyleDerZweite]

Summary

• Collapses the one-implementation backend trait into MockBackend and removes 13 unused bridge commands plus their stubs.
• Removes unproduced event variants and orphan event types, then regenerates src/bindings.ts.
• Removes the unused async-trait and direct serde_json dependencies plus the unused MockBackend default implementation.
• Reduces TypeScript and Tailwind configuration to settings used by the current client.
• Inlines the default TypeScript binding configuration and updates compatibility documentation.

Skipped findings

• None. All seven audit findings matched current callers, event producers, and configuration use.

Compatibility

• The generated command API now exposes restoreSession, listSpaces, listRooms, loadTimeline, and sendMessage.
• The event union now exposes Timeline and Typing.
• Rust users of ChatBackend must use MockBackend directly. Restore an operation with backend behavior, a Tauri wrapper, and a UI caller. Add a shared trait when a second backend needs it.

Checks

• Rustfmt check using edition 2021 on every changed Rust source
• Cargo tests with workspace scope
• Frontend typecheck and production build with bun run build

[Copilot]

Pull request overview

Refactors the client by shrinking the Rust↔UI contract surface to only what the current prototype uses, removing the previously generic backend trait seam, and aligning generated TypeScript bindings + configuration with that reduced API.

Changes:

• Removes the ChatBackend trait and prunes unused commands/events/types across Rust core, Tauri bridge, and generated src/bindings.ts.
• Implements/extends mock-only functionality for replies, edits, redaction, and reactions, including expanded Rust tests for these behaviors.
• Simplifies frontend configuration (TypeScript/Tailwind) and updates compatibility documentation to reflect the prototype-only contract.

Reviewed changes

Copilot reviewed 12 out of 16 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tsconfig.json	Simplifies TS compiler options to only what the current client uses.
tailwind.config.js	Reduces Tailwind config to minimal settings used by the UI.
src/bindings.ts	Regenerated bindings after pruning commands/events/types.
src/App.tsx	Adds reply/edit/delete/reaction UI flows and improves async error handling and race-safety when switching rooms.
src-tauri/src/lib.rs	Removes trait-based backend indirection; exposes only the remaining commands and inlines TS export config.
src-tauri/Cargo.toml	Drops unused serde_json dependency.
src-tauri/build.rs	Formatting-only change.
README.md	Updates project framing to clarify prototype/mock-only behavior and future Matrix replacement steps.
docs/DATA_INTERFACE.md	Updates contract documentation to match the reduced API/event surface and prototype scope.
core/src/model.rs	Removes unused session/login and membership/member types; minor formatting.
core/src/mock.rs	Implements reply/edit/redact/reaction behavior and adds substantial test coverage.
core/src/lib.rs	Removes backend trait module/re-exports and narrows public exports.
core/src/event.rs	Prunes events to Timeline and Typing only and removes unused event-related types.
core/src/backend.rs	Deletes the unused ChatBackend trait module.
core/Cargo.toml	Drops unused async-trait dependency.
Cargo.lock	Removes async-trait and serde_json from the lockfile dependency graph.

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