Allow API key for Ollama connections

[shimmyshimmer]

Fixes #7163

Problem

The Connections form (Settings > Connections) hid the API key field for the Ollama preset. customPresetSkipsApiKeyField in external-providers.ts returned true for both ollama and llama_cpp, so there was no way to enter a key. Ollama cloud requires one, so cloud endpoints could only be used through a Custom connection workaround.

Fix

Everything downstream was already key capable: the provider registry defines Authorization: Bearer auth for ollama, and model listing, connection tests, and chat requests all forward a stored key. The only blocker was the hidden input.

• external-providers.ts: remove ollama from customPresetSkipsApiKeyField, only llama_cpp still hides the field
• chat-providers-dialog.tsx: update the stale comment at the showApiKeyField computation
• providers.py: update the ollama registry note from "no API key" to "API key optional (required by Ollama cloud)"

Since Ollama is a custom provider type, the existing form logic already labels the field as optional, stores the key locally per connection, and removes a stored key when the field is saved empty.

Behavior

• Ollama cloud: set Base URL to https://ollama.com/v1 and paste a key, requests send Authorization: Bearer <key>
• Local Ollama: leave the field empty, no auth header is sent (avoids the httpx "Illegal header value" failure on an empty Bearer), so existing setups are unaffected

Testing

• tsc -b passes
• Verified against the backend client: an ollama provider with a key produces Authorization: Bearer <key>; with an empty key no auth header is present
• Rebuilt the frontend and confirmed the Connections form shows "API key (optional)" for Ollama

[gemini-code-assist]

Code Review

This pull request updates the Ollama provider configuration to support an optional API key, which is required for Ollama cloud deployments. The frontend has been updated to display the API key field for Ollama (making it optional for local servers and required for cloud), whereas previously it was hidden. There are no review comments, so I have no feedback to provide.

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