feat(desktop): embedded live preview for project repos (render `web` URL in project pane)

### Feature request: embedded live preview for project repos (web URL)

**Platform:** Buzz Desktop
**Repo tracker:** https://github.com/block/buzz

### Summary
When a git project is opened in Buzz's project pane, the view shows the repo's source README. Add an option to render the project's declared `web` URL as an embedded live preview (iframe) instead of / in addition to the README, so a user can see the deployed site without leaving Buzz.

### Background / context
A repo announcement (NIP-34) already carries a `web` tag pointing at the live site (e.g. a GitHub Pages deployment at `https://bitpopart.com`). The data needed to render a preview is therefore already present — the client just currently ignores `web` in the project detail view and shows only the README.

### Proposed behavior
- In the project detail view, offer a toggle/tab: **README** | **Live preview**.
- "Live preview" renders the `web` URL in an embedded frame (sandboxed, CSP-restricted, with an explicit "open in new tab" affordance for the full site).
- Falls back gracefully when the `web` tag is absent (show README only) or when the site blocks framing (show a click-to-open link).

### Why
Creators want to confirm their deployed site matches the repo without switching apps/browsers. This makes Buzz a genuine self-contained view of a project: code (repo), docs (README), and the running artifact (live preview) in one place.

### Notes
- Original request came from a user wiring their GitHub Pages site (`bitpopart.com`) to a Buzz project bound to a channel.
- Consider reuse for branch/PR previews later (render a deployed environment for a given ref).