feat: add setting to control default expanded state of thinking blocks

[codeOwnerX]

Problem / Motivation

Currently, thinking (reasoning) blocks in assistant messages are always collapsed by default. Users must click each block individually to view the reasoning content. For users who want to review or audit the model's reasoning process, this requires repetitive manual interaction on every response.

Proposed Solution

Add a user preference setting to control whether thinking blocks render expanded or collapsed by default:

• Default behavior (no change): collapsed
• New option: always expanded by default

The toggle should persist across sessions via localStorage.

Implementation Location

components/MessageView.tsx, ThinkingBlock component:

function ThinkingBlock({ block, duration, sessionId, entryId, blockIndex }) {
  const [expanded, setExpanded] = useState(false); // currently hardcoded
  ...
}