feat(web): add /new composer thread command

[binbandit]

Summary

• add /new as a composer slash command and suggestion to start a fresh thread in the current project
• share project draft-thread routing between the sidebar and chat composer so new-thread entry points stay consistent
• add parser, helper, and browser coverage for the new thread flow

Testing

• bun fmt
• bun lint
• bun typecheck
• bun run --cwd apps/web test src/composer-logic.test.ts src/lib/projectDraftThreads.test.ts
• bun run --cwd apps/web test:browser src/components/ChatView.browser.tsx

UI

Note

Add /new slash command to the composer to open a project draft thread

• Adds /new to the composer's slash command menu and standalone submission handling in ChatView.tsx; selecting or submitting the command navigates to a new or existing project draft thread.
• Extracts shared draft-thread selection logic into openOrReuseProjectDraftThread in projectDraftThreads.ts, which prefers a stored draft, falls back to reusing the current thread if it belongs to the project, or creates a new thread with default runtime and interaction modes.
• Refactors Sidebar.tsx handleNewThread to delegate to the same shared utility, removing duplicated inline logic.
• Extends ComposerSlashCommand type and parseStandaloneComposerSlashCommand in composer-logic.ts to recognise 'new' as a valid command.

^{Macroscope summarized 24f75f2.}

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