FIX Persist Chat Markdown display preference

[romanlutz]

Description

The Chat raw/Markdown switch previously used component-local state, so navigating to another view or reloading unexpectedly reset users to raw mode. This change persists explicit raw and markdown choices in localStorage, initializes the switch lazily on mount, and keeps raw mode as the default for first-time users.

Invalid or unavailable browser storage safely falls back to raw mode without changing the existing Markdown renderer, sanitization, conversation-switch behavior, or accessibility contract.

Tests and Documentation

• Focused Jest coverage for default behavior, stored initialization, invalid/unavailable storage, and persistence across remounts
• Combined ChatWindow and conversation-export suites: 130 tests passed
• Frontend lint passed
• Frontend type-check passed
• Focused Playwright Chat -> History -> Chat navigation and reload check passed
• Documentation: N/A; this preserves the existing switch behavior while making the preference durable
• JupyText: N/A; no documentation notebooks changed

Before and after

The clip first shows the preference reset on origin/main, then the fixed navigation and reload behavior.

markdown-preference-before-after.webm