Fix SPA routing for production builds - serve index.html on 404

[Copilot]

Summary

Production builds (pnpm build) returned 404 for client-side routes like /login, while dev mode worked correctly. Starlette's StaticFiles with html=True doesn't properly fallback to index.html for all SPA routes.

Solution: Override get_response in NoCacheStaticFiles to catch 404 exceptions and serve index.html when html=True:

async def get_response(self, path: str, scope: Scope) -> Response:
    try:
        return await super().get_response(path, scope)
    except HTTPException as exc:
        if exc.status_code == 404 and self.html:
            return await super().get_response("index.html", scope)
        raise

This ensures all unmatched routes serve the SPA entry point, allowing client-side routing to handle navigation.

Related Issues / Discussions

N/A

QA Instructions

1. Build frontend: cd invokeai/frontend/web && pnpm build
2. Start backend with production build
3. Navigate directly to /login or any client-side route via URL
4. Verify SPA loads instead of returning "Not found"
5. Verify existing static assets (JS, CSS, images) still load normally

Merge Plan

N/A

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable) - No existing test infrastructure for static file serving
• ❗Changes to a redux slice have a corresponding migration - N/A
• Documentation added / updated (if applicable) - N/A
• Updated What's New copy (if doing a release after this PR)

Original prompt

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This section details on the original issue you should resolve

<issue_title>[bug]: "Not found" errors in multiuser mode</issue_title>
<issue_description>### Is there an existing issue for this problem?

• I have searched the existing issues

Install method

Invoke's Launcher

Operating system

Linux

GPU vendor

Nvidia (CUDA)

GPU model

No response

GPU VRAM

No response

Version number

feature/multiuser branch

Browser

No response

System Information

No response

What happened

**IMPORTANT: working in the branch feature/multiuser.

When launching invoke with the frontend using pnpm dev, the application functions as expected. On the first launch, it asks the administrator to set up an account and then redirects to /login to obtain the login credentials.

However, when launching invoke with the frontend built using pnpm build, after collecting the administrator account information, it redirects to /login and the application returns a white page with the text "Not found."

What you expected to happen

I expect pnpm dev and pnpm build to produce functionally identical user experiences.

How to reproduce the problem

No response

Additional context

No response

Discord username

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [bug]: "Not found" errors in multiuser mode #57

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