refactor(frontend): Update Supabase and backend API management

[kcze]

Currently there are random issues (logout, auth desync) and inconveniences with how Supabase and backend API works.
Resolves:

• Fix Supabase auth sync issues #9006
• Automatic server-side/client-side backend API client usage #8912

Changes 🏗️

This PR streamlines how the Supabase and backend API is used to fix current errors with auth, remove unnecessary code and make it easier to use Supabase and backend API.

• Add getServerSupabase for server side that returns SupabaseClient.
• Add Spinner component that is used for loading animation.
• Remove redundant useUser, user is fetched in useSupabase already.
• Replace most Supabase create*Client to getSupabaseServer and useSupabase.
• Remove redundant AutoGPTServerAPI class and rename BaseAutoGPTServerAPI to BackendAPI and use it instead.
• Remove SupabaseProvider context; supabase caches internally what's possible already.
• Move useSupabase hook to its own file and update it.

Helpful table

Next.js usage	Server	Client
API	new BackendAPI();	new BackendAPI();* or useBackendAPI()
Supabase	getServerSupabase();	useSupabase();
user, user.role	getServerUser();**	useSupabase();

* BackendAPI automatically chooses correct Supabase client, so while it's recommended to use useBackendAPI(), it's ok to use new BackendAPI(); in client components and even memoize it: useMemo(() => new BackendAPI(), []).

** The reason user isn't returned in getServerSupabase is because it forces async fetch but creating supabase doesn't, so it'd force getServerSupabase to be async or return { supabase: SupabaseClient, user: Promise<User> | null }. For the same reason useSupabase provides access to supabase immediately but user may be loading, so there's isUserLoading provided as well.

Checklist 📋

For code changes:

• I have clearly listed my changes in the PR description
• I have made a test plan
• I have tested my changes according to the test plan:
  • ...

Example test plan

• Create from scratch and execute an agent with at least 3 blocks
• Import an agent from file upload, and confirm it executes correctly
• Upload agent to marketplace
• Import an agent from marketplace and confirm it executes correctly
• Edit an agent from monitor, and confirm it executes correctly

For configuration changes:

• .env.example is updated or already compatible with my changes
• docker-compose.yml is updated or already compatible with my changes
• I have included a list of my configuration changes in the PR description (under Changes)

Examples of configuration changes

• Changing ports
• Adding new services that need to communicate with each other
• Secrets or environment variable changes
• New or infrastructure changes such as databases

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