Landing polish + share preview, and a one-click browser extension

#3 Landing / appeal / shareability:
- Added a 3-step "How it works" strip so the concept lands before the
  mechanics.
- Added Open Graph + Twitter Card meta tags and a generated 1200x630
  share image (static/og.png) so the link shows a real branded preview
  when shared on LinkedIn/Slack/etc. instead of a bare URL. Also added
  an emoji favicon.

#2 Browser extension (extension/):
- MV3 extension that does exactly what the bookmarklets do, but as a
  one-click toolbar button - removes the drag-to-bookmarks-bar step
  that loses non-technical users. depop.js/vinted.js are the exact
  tested bookmarklet bodies (base URL hardcoded to prod), bundled so
  it passes Chrome's no-remote-code rule; background.js injects the
  right one based on the active tab's domain.
- Loads unpacked in seconds for immediate use/sharing; extension/README
  covers that plus Chrome Web Store publishing (the $5 dev-account step
  is the user's to do).
- app.py notes that the extension carries generated copies of the two
  scripts, to keep them in sync.

The extension's only untested-by-me code is the tiny standard MV3
manifest + background worker (JSON + JS both syntax-validated); the
actual scraping logic inside is the same code verified live earlier.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>