feat: add dashboard AI chat with TanStack multi-provider runtime

[CarlosZiegler]

Summary

• add a new authenticated chat page at /(dashboard)/chat
• add AI Chat entry to the dashboard sidebar
• build chat UI with AI Elements components (Conversation, Message, PromptInput, ModelSelector)
• keep chat transport on TanStack AI + /api/chat SSE endpoint
• enable runtime provider/model selection (OpenAI, Anthropic, Gemini)
• sync AI env vars for Gemini support in app env + CLI env template
• update landing AI snippets to reflect multi-provider runtime

Notes

• backend chat remains TanStack REST endpoint (not oRPC)
• AI Elements is used for UI composition

Files touched

• apps/start-template/src/routes/(dashboard)/chat/index.tsx
• apps/start-template/src/components/app-sidebar.tsx
• apps/start-template/src/lib/chat/use-resumable-chat.ts
• apps/start-template/src/routes/api/chat/index.ts
• apps/start-template/src/lib/env.server.ts
• packages/cli/src/templates/env.template.ts
• apps/start-template/src/routes/index.tsx

Validation

• targeted TypeScript checks for changed files passed