Fix client-side crash when vLLM server is unavailable

[Copilot]

Application crashes with "a client-side exception has occurred" when the vLLM backend is unreachable. Three issues cause this:

Changes

• src/lib/token-counter.ts: getTokenLimit now returns default value (4096) on error instead of throwing unhandled exceptions
• src/components/chat/chat-topbar.tsx:
  • Validate data.data exists before calling .map() on API error responses
  • Fix toast.error(error as string) → toast.error(error instanceof Error ? error.message : String(error)) — Error objects aren't renderable by React
• src/components/chat/chat-bottombar.tsx: Add .catch() handler for getTokenLimit promise

Before

After

Original prompt

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This section details on the original issue you should resolve

<issue_title>Application error: a client-side exception has occurred (see the browser console for more information).</issue_title>
<issue_description>

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Application error: a client-side exception has occurred (see the browser console for more information). #8

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