docs(capture-harness): rework react theming screenshot pipeline

- Share one retina context across captures (viewport 1280x1200 @1x) so
  localStorage and auth tokens persist and the message list has time
  to populate before each screenshot. Previous per-capture contexts
  raced an empty channel list on themed variants.
- scrollToLatest() before every screenshot so both users' latest
  bubbles land in the viewport.
- Explicitly create the target channel with both members + a friendly
  name before seeding, and truncate it so reruns don't stack
  duplicate messages.
- waitForChatUI() now also waits for at least one rendered .str-chat__li.
- Headless by default (HEADED=1 to override).
- Extract the CSS override blocks to theming-variants.mjs so the
  harness and the doc (01-themingv2.md) stay in lockstep.
- Fix ASSETS_DIR to the real docs-content tree
  (../../../../docs/data/docs/chat-sdk/react/v14-latest/_assets) —
  previous path resolved to a non-existent directory.
- Drop obsolete capture flows (link-attachment text color, layout-only
  CSS) — neither applies to v14.
- Rewrite the seed conversation: two-user dialogue with emojis,
  markdown (**bold**, *italic*, `code`), GitHub URL preview, images,
  and richer reaction variety.
- Add @playwright/test as a devDep of examples/vite.

Author: oliverlaz