Feat/Add multiple provider keys in BYOK onboarding (task 5)

[prakhar1605]

What

Lets users add multiple provider keys during BYOK onboarding. After saving a
provider, they can either finish or add another provider's key; all saved keys
persist together and their models are available in the chat picker. (Addresses
BYOK task 5. Builds on #629 — please review/merge that first.)

Why

Previously onboarding saved one provider and immediately advanced, so a user could
only configure a single provider during setup. Some users want several (e.g. an
Anthropic key for chat and a Gemini key as well).

How

• The single-key path is unchanged: enter a key → "Test & Continue" → saves and
  advances in one click.
• Refactored the save logic into a shared testAndSaveActiveProvider() helper with
  two wrappers: handleTestAndSaveLlmConfig (save + advance, unchanged behavior) and
  handleTestAndAddAnother (save + stay).
• Added a connectedFlavors set tracking which providers have been saved this session;
  connected provider cards show a checkmark.
• "Save & add another" saves the current provider, then switches the form to the next
  provider the user hasn't connected yet (so the form is fresh and the buttons re-enable
  for the next key).
• The primary button becomes a plain "Continue" once at least one provider is connected,
  so the user can finish even from an empty form (no dead-end).
• No backend change needed — repo.setConfig already merges each provider into the
  config's providers map without overwriting the others.

Files changed

• apps/x/apps/renderer/src/components/onboarding/use-onboarding-state.ts
• apps/x/apps/renderer/src/components/onboarding/steps/llm-setup-step.tsx

How to test

1. Sign out of Rowboat (BYOK mode), reset onboarding.
2. Pick Anthropic → enter key → "Save & add another". The Anthropic card shows a ✓,
   the form switches to a fresh provider, and the primary button reads "Continue".
3. Pick Gemini (or another provider) → enter key → "Test & Continue" to finish.
4. Verify ~/.rowboat/config/models.json → the providers map contains BOTH providers,
   each with its own key + models.
5. Single-key path unchanged: enter one key → "Test & Continue" → finishes in one click.

Known follow-ups

• (Same as Simplify BYOK onboarding to provider + key (tasks 4 & 6) #629) Provider model lists include non-chat models (e.g. Gemini's tts /
  image / video / embedding entries). The model-filtering follow-up will clean these
  up across all providers.