Use the active project for Command+N thread creation

[ryanbbrown]

Fixes #1618

Summary

• Select the active thread's project before Command+N opens the new-thread composer.
• Preserve the stored composer project on routes that do not belong to a project.

Root cause

The thread.new command opened the composer without updating its stored project selection. The composer could therefore reuse a project selected during an earlier session instead of the project for the active thread.

Validation

• Focused Vitest file: 2 tests passed.
• pnpm exec turbo run typecheck --filter=@bb/app
• git diff --check
• Independent review passed with Codex and GLM.

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