feat(chat): render assistant markdown instead of raw ### and **

[SecureCloudGroup]

Why

The chat rendered the assistant's reply as plain text (chat/+page.svelte:675 → {entry.content}), so every answer displayed the model's raw markdown — literal ###, **bold**, * bullets, and code fences. Models answer in markdown by default, so the product's primary surface looked broken next to any other chat UI.

marked was already in the repo, but only used at build time for the Help docs — never at runtime.

What

• lib/markdown.ts — marked → DOMPurify. Assistant output is untrusted: a reply can quote a fetched web page, an ingested PDF, or an email verbatim, so a crafted document could smuggle <img onerror=…> or a javascript: link into the answer. Injected script would run in the app origin, which holds an unlocked session — so the model's HTML is sanitized before it ever reaches the DOM (no <script>, no on* handlers, no javascript: URLs, no iframes/inline styles). Real links open in a new tab with no opener reference.
• lib/Markdown.svelte — renders the assistant bubble, re-renders safely on each streaming delta, and attaches a copy button to every code block.
• app.css — markdown block styles. .bubble uses white-space: pre-wrap for verbatim text, which had to be reset for rendered HTML or every paragraph gained a phantom blank line. Wide tables and long code lines scroll inside the bubble; the page never scrolls sideways.
• User messages and error bubbles stay verbatim (unchanged).

Verification

• 9 new tests — renders headings/bold/lists/tables/fenced code; survives half-finished markdown (a reply still streaming); strips <script>, on* handlers, javascript: links and iframes; forces target=_blank + noopener.
• Web suite 63 passed, svelte-check 0 errors / 0 warnings, build green, SPA bundle rebuilt.
• Rendered the actual reply from the earlier session (the one that showed ### **Key Details:**) through the real pipeline + real stylesheet in headless Chromium: headings, nested bullets, inline code, a GFM table, a blockquote, a code block and a link all render correctly.

Part of Cycle 1 (W0.1) of the Knowledge/Install plan — "present answers in a consumable way."