Add target-app selection and routing support

[Fayiz1]

feat(web): Support dual Antigravity engines and add chat auto-scroll ResizeObserver

Detailed Changes:

1. Chat Auto-Scroll Stabilization (ResizeObserver):

   • Resolved a critical bug where long chats would jump/stay scrolled to the top on load.
   • Introduced a ResizeObserver on the inner messages container to detect asynchronous height increases (e.g. text expanding, Markdown compiling, images loading) and snap the scroll position to the bottom if the user was already near the bottom.
   • Restructured layout references (innerRef and scrollRef) in ChatPanel.tsx to handle the resize events.

2. Dual-Target Engine Support (Antigravity & Antigravity IDE):

   • Enabled target routing selection in the UI, allowing the Porta web app to switch between both the standard antigravity Language Server and the Desktop antigravity-ide Language Server.
   • Ensured headers (x-porta-target-app) and WebSocket parameters are correctly passed so that step updates and message submissions route to the correct LS process.

3. Background Mobile Load & Caching Optimization:

   • Implemented sessionStorage caching for conversation trajectories in useStepsStream.ts to instantly restore message histories.
   • Eliminated the jarring blank "loading skeleton" flash when users perform a pull-to-refresh on mobile devices or reload on unstable network connections.
   • Set up silent background polling to merge and sync new steps seamlessly.