Persist Ask thread sidebar width + render markdown code blocks with syntax highlighting

[jstuart0]

Goal

Two small improvements to the `/ask` page that together make long sessions much more usable.

Part A — Resizable, persisted sidebar

Today the thread sidebar is a fixed `w-60`. For long thread titles that's cramped; for short titles it's wasted space.

• Make the sidebar resizable with a drag handle on the right edge.
• Persist the width in `useUiPrefsStore` (new field `askSidebarWidth`, default 240px).
• Clamp between 180 and 480 px.

Part B — Syntax-highlighted code blocks

Assistant replies render through `MarkdownContent`, which shows fenced ```ts / ```bash blocks as plain `

`. Add real syntax highlighting.


Install `shiki` (tree-shakeable, already modern).
Wire it into `MarkdownContent.tsx` via a custom `code` component renderer.
Support at least: `ts`, `js`, `tsx`, `sh`, `json`, `yaml`, `sql`, `python`, `go`.
Pick a theme that reads in both light and dark (e.g. `github-dark-dimmed` / `github-light` via the existing theme toggle).

Acceptance criteria

 Sidebar resizes smoothly, width persists across reloads.
 Fenced code blocks in assistant replies render with colors that match the current theme.
 No runtime flash — async-load highlighter only when an Ask reply is rendered on-screen.