Add self-hosted local LLM provider support

[manueljpconde]

Summary

• Adds a server-configured local:server LLM provider backed by OpenAI-compatible Chat Completions streaming.
• Exposes backend-derived local provider availability/capabilities without exposing local endpoint secrets.
• Gates local tool usage unless LOCAL_LLM_SUPPORTS_TOOLS=true, and keeps local out of tabular preferences unless tool support is enabled.
• Adds backend tests for provider routing, local config parsing, SSE parsing, and no-tool local streaming.
• Documents Ollama and LM Studio self-hosting examples.

Verification

• npm test --prefix backend
• npm run build --prefix backend
• npx eslint 'src/app/(pages)/account/models/page.tsx' 'src/app/components/assistant/ModelToggle.tsx' 'src/app/lib/modelAvailability.ts' 'src/contexts/UserProfileContext.tsx' 'src/app/lib/mikeApi.ts' 'src/app/components/shared/ApiKeyMissingModal.tsx' from frontend/
• NEXT_PUBLIC_SUPABASE_URL=https://example.supabase.co NEXT_PUBLIC_SUPABASE_PUBLISHABLE_DEFAULT_KEY=placeholder npm run build --prefix frontend

Note: full npm run lint --prefix frontend still fails on pre-existing lint issues outside this change set.

[manueljpconde]

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