Summary

Adds a "New Chat" button to the app builder chat pane header, allowing users to start a fresh AI session within an existing project. The new session gets a clean LLM context while preserving the project's full codebase and previous session history.

User flow:

1. User clicks the SquarePen icon in the chat header (disabled while streaming)
2. Existing session history stays visible; a "New chat session — Your code is preserved" placeholder is appended below
3. Clicking the button again cancels and restores the session to its previous visual state
4. Sending a message creates a new cloud agent session via forceNewSession: true on the existing sendMessage endpoint; the old session collapses into an expandable history block via the existing handleSessionChanged machinery

Architecture: The forceNewSession flag is handled entirely at the ProjectManager level — it calls tRPC directly and then delegates to the existing handleSessionChanged handler. Zero changes to V1/V2 session or streaming code. No DB migration needed (the reason column is freeform text).

Key implementation details:

• pendingNewSession (in ProjectManager store) tracks whether the user has clicked "New chat" but not yet sent
• awaitingNewSession (in AppBuilderChat) bridges the one-render gap between pendingNewSession flipping to false on send and the new session object arriving in state — implemented with useState + useEffect rather than a render-time ref mutation
• requestNewSession/cancelNewSession set session.info.ended_at optimistically so the current session collapses into a history block while the user composes their message
• Layout heights changed from h-dvh to h-[calc(100dvh-3.5rem)] across AppBuilderPage, AppBuilderLanding, and ProjectLoader to account for the top navigation bar

Verification

• Traced tRPC input flow from Zod schema → service type → service logic → both personal and org routers
• Confirmed forceNewSession path in app-builder-service.ts bypasses shouldCreateNewSession and uses the existing createV1Session/createV2Session helpers with the new user_initiated reason
• Confirmed pendingNewSession state resets correctly on send, cancel, and session creation
• pnpm typecheck passes; oxfmt format check passes

Visual Changes

Reviewer Notes

• The SquarePen button is a toggle: click to enter pending mode, click again to cancel. It is disabled while isStreaming is true so users cannot trigger it mid-stream.
• The sendMessageAsNewSession function in ProjectManager.ts calls tRPC directly (rather than going through the session layer) because the existing session must be ended server-side before the new one is created. The response triggers handleSessionChanged, which reuses all existing session-change UI logic.
• AppBuilderSessionReason.UserInitiated ('user_initiated') is added to the schema constant but requires no migration since the column is text NOT NULL with no DB-level constraint on values.
• The previous behaviour replaced the entire chat area with a blank placeholder when pendingNewSession was true. This PR keeps session history visible and only appends the placeholder, which required the awaitingNewSession state to prevent a flicker when pendingNewSession clears on send.