fix: keep chat history actions visible

[klioen]

Summary

Give Chat history more room and keep its delete action visible when a session title is too long for the popover row.

Changes

• Increase the responsive popover width from 360px to 420px.
• Let the primary history item flex into the row's remaining width instead of claiming width: 100% beside the delete button.
• Keep the item shrinkable with min-width: 0, preserving the existing ellipsis behavior for long titles.
• Keep the delete action non-shrinking and add a CSS contract regression test.
• Document the user-visible fix in CHANGELOG.md.

Risk and boundaries

• Regression risk: Low. This is a localized CSS flex-layout correction with no behavior, persistence, IPC, or security-boundary changes.
• Rollback: Revert this PR.

Verification

pnpm vitest run src/styles/themeTokens.test.ts src/features/chat/ChatHistory.test.tsx
# 2 files, 15 tests passed

pnpm check
# 68 files, 454 tests passed; release tests passed

git diff --check HEAD
# passed

• Tests cover the layout contract.
• No dependency or generated artifact changes.
• No credential, privacy, filesystem, Git, or IPC boundary changes.