Add Google Sign-In integration for SolveIt-hosted apps

[ncoop57]

Summary

Adds a new solve_auth module that provides Google Sign-In functionality for apps hosted on SolveIt.

Changes

• New setup_solve_signin() function to add authentication routes to FastHTML/Starlette apps
• sub_from_signin() to decode and validate JWT tokens from the auth callback
• Custom SolveSigninError exception for auth failures
• Handles redirect flow through SolveIt's auth service at auth.solve.it.com

Minimal fasthtml app using this:

from dialoghelper.solve_auth import *
from fasthtml.common import *
from fasthtml.jupyter import *

app, rt = fast_app()
srv = JupyUvi(app, port=8000)
setup_solve_signin(app)

@rt
def index(): return Titled('Auth Test', A('Sign In with Google', href=solve_signin_rt, cls='button'))

@rt
def signin_completed(session, signin_reply: str=''):
    session['auth'] = sub_from_signin(session, signin_reply)
    return Titled('Signed In!', Pre('Sub ID: ' + session['auth']))

I've tested it out on prod and it worked as expected.

[ncoop57]

@jph00 how should the deps for this be included? Should they be optional?