fix: restore api keys tab and apply session key to pipeline default m…

[harz05]

Fixes #475

Problems

1. The API Keys tab in chat Settings was hidden (<!-- commented out -->) due to a known bug where the session key was ignored.

2. When a user saved a session API key but did NOT explicitly select a provider in the UI (i.e. used the pipeline default model), the session key was never looked up and the deployment admin key was always used instead, reverting any per-user override.

Fix

index.html — Uncommented the API Keys nav button and section. Updated the description to clarify keys live in the browser session only.

app.py — In get_chat_response_stream(), extended the session key lookup to also cover the no-provider-selected case: when no explicit provider comes in from the frontend, the code now reads the pipeline's active model from dynamic_config (or falls back to services.chat_app.default_provider), checks if the user has a session key for that provider, and injects it — so the BYOK key is used transparently even with the default pipeline model.

Tested

• Set a fake session key (sk-fake999) → sent a message → got an auth error from OpenAI confirming the session key was injected
• Cleared session key → chat worked normally with env key
• API Keys tab visible and functional in Settings modal