De-jargon user-facing model/API-key copy

Frontend-only copy cleanup. No identifiers, file names, MIME types,
localStorage keys, env vars, or backend contracts were renamed -
those carry behavior and were intentionally left alone.

User-facing changes:
- Account settings tab "Models & API Keys" → "AI configuration".
- /account/models page:
  - "Model Preferences" → "Default assistant"
  - "Tabular review model" → "Assistant for tabular reviews"
  - "API Keys" → "AI service keys"
  - Body copy rewritten to plain English ("GaryOSS connects to an
    outside AI service to answer questions...").
  - "Anthropic (Claude) API Key" → "Anthropic (Claude) key"
  - "Google (Gemini) API Key" → "Google (Gemini) key"
  - Tooltip "Add a {vendor} API key to use this model" →
    "Add a {vendor} key in AI configuration to use this assistant"
  - Placeholder "Select a model" → "Select an assistant"
- ApiKeyMissingModal:
  - Title "API key required" → "GaryOSS isn't configured yet"
  - Body softened to "GaryOSS isn't connected to {service} yet.
    Open AI configuration to add a key."
  - CTA "Go to account settings" → "Open AI configuration"
- ModelToggle:
  - title "API key missing for selected model" → "No key set for
    this assistant"
  - title "Choose model" → "Choose assistant"
  - aria-label "API key missing" → "No key set"
  - Fallback label "Model" → "Assistant"

GaryOSS branding is already consistent in README, layouts, and
schema header. No backend files touched. License unchanged.

https://claude.ai/code/session_019pRkhcGDRKQWHjzAnV5yCL