Add assistant message starring and fix Svelte state handling

[jcrabapple]

Summary

This PR adds support for starring assistant messages in the chat view and fixes a Svelte state-handling issue related to message.starred in src/routes/chat/[id]/message.svelte.

Changes

• Add a star/unstar toggle button for assistant messages in src/routes/chat/[id]/message.svelte.
• Track isStarred using $state plus a $effect that reacts to message.starred instead of capturing the initial message value in the state initializer.
• Wire the star toggle to api.messages.setStarred with optimistic UI updates and rollback on failure.
• Invalidate api.messages.getAllFromConversation after a successful update so other consumers stay in sync.

Rationale

Previously, the starred state logic referenced message directly inside the $state initializer, which caused Svelte to warn that the reference only captures the initial value. This PR moves that logic into a $effect, ensuring that isStarred always reflects the latest message.starred value.

The new star toggle provides a quick way for users to flag assistant messages as important, while keeping the UI responsive and consistent with server state.

Testing

• Started the dev server with bun --bun run dev.
• Opened a chat and generated assistant messages.
• Verified that:
  • The star icon appears only on assistant messages with non-empty content and no error.
  • Clicking the star toggles its visual state between filled (starred) and muted (unstarred).
  • The starred state persists after a refresh (server round-trip).
  • No Svelte warnings about local state references to message are emitted at build/runtime.

Co-Authored-By: Warp agent@warp.dev