fix(web): open off-site links in a new tab

[hmseeb]

Problem

In the web client, links inside markdown-rendered content navigate the current
tab. Click a link in a repo's README, a previewed .md file, or the Terms /
Privacy documents on the invite page, and you're pulled out of Buzz entirely —
losing the repo you were browsing or, worse, the invite you were mid-way
through accepting.

Hand-written anchors already got this right (View on web, Download it now).
Markdown was the gap, and it's the surface most likely to contain arbitrary
third-party links.

Fix

A shared Markdown component (web/src/shared/ui/markdown.tsx) that adds
target="_blank" + rel="noopener noreferrer" to cross-origin http(s) links
only. The three existing call sites now use it instead of each configuring
react-markdown themselves, which also de-duplicates the remarkGfm wiring.

Deliberately left navigating in place:

• Same-origin paths and in-page anchors — normal in-app navigation.
• Custom schemes, notably the buzz:// deep links behind Open in Buzz and
  Accept invite in Buzz. Opening those in a new tab hands off to the desktop
  app and strands an empty tab behind it.

No new dependency — rehype-external-links would do this, but it isn't worth a
package for an eight-line predicate.

Test

web/tests/e2e/smoke.spec.ts gains a case that serves a join policy whose
terms markdown contains an off-site link, an in-app path, and an in-page
anchor, then asserts only the first opens in a new tab. Confirmed failing
before the fix (Expected "_blank", Received ""), passing after.

Full web smoke suite: 7/7 pass. pnpm check and pnpm typecheck clean.