Add Mermaid Markdown rendering and standalone image viewer

[notsuhas]

Adds Mermaid diagrams to Markdown, a standalone image viewer, index.md landing pages, and AVIF MIME support.

• Mermaid renders locally with a locked strict policy, enlarge/close controls, and readable source fallback on errors.
• Images support zoom, fit, open original, and download. Existing content access checks stay in place.
• No image processing, caching redesign, pinpoint comments, or new infrastructure.

Verification: API tests (1,349), web tests (548), script/npm tests, typecheck, lint, formatting, Go tests, web build, and content-worker dry run passed. Separate source review found no blocking issues.

Browser checks used the built app and production CSP with isolated local fixtures: image zoom/reset/download, Markdown diagrams, invalid-source fallback, strict-policy cases, and enlarge/Escape behavior. Desktop and 390px layouts were inspected; a dialog overflow found on mobile was fixed and rechecked. Physical devices and live production were not tested. The fixture omits realtime sockets, so realtime collaboration was not exercised.