Render Markdown in browser chat messages

[AgentStart1]

Summary

Render Markdown in browser chat messages instead of showing formatting syntax as plain text. Support headings, emphasis, lists, quotes, fenced code, tables, links, and images while keeping the copy button's output as the original Markdown.

Parse each incoming message once in the existing Worker with a locally bundled markdown-it 15.0.2. Raw HTML stays disabled, dangerous URL validation remains enabled, and sender-supplied HTML fields are ignored. Links open separately with noopener noreferrer. Include upstream license notices and a reproducible vendoring script.

Keep long messages inside the scrollable message log so the composer stays visible on phones. Code blocks and tables scroll horizontally without widening the page.

This builds directly on merged PR #32 and contains only the Markdown increment. No CI workflow changes.

Validation

Completed locally during implementation:

• 10 web Host and Markdown tests passed, covering formatting, unsafe HTML/URLs, sender-supplied HTML, and preservation of original text.
• Chromium desktop/mobile regression passed, including Markdown rendering, raw-text copy, long code blocks, composer visibility, untrusted payloads, reconnect, and existing file/login flows.
• :app:testDebugUnitTest --tests '*WebPagesTest' passed, including real Ktor serving of the parser modules with JavaScript MIME types.
• :app:assembleDebug passed.

Browser interaction checks use mock HTTP/SSE/WebSocket endpoints; real Ktor resource serving is checked separately on the JVM. Installed-device end-to-end testing and Alpha/Release builds were not run for this change.