prelaunch polish: bigger social wordmark + Python 3.14 disclaimer (#13)

* fix: make wordmark dominate banner_social.png instead of floating in dark padding

The 1280x640 social card had the wordmark capped at 70% height and 85%
width, leaving roughly a third of the canvas as empty dark margin. When
platforms cropped to tighter aspect ratios for link previews, the
wordmark looked tiny. Push the fill to 92% width so the wordmark reads
clearly when shared.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* docs: add Python 3.11-3.13 disclaimer after install step

3.14 unsupported until upstream Crawl4AI relaxes its lxml~=5.3 pin
(unclecode/crawl4ai#1903). Without a callout, users on 3.14 hit a
cryptic lxml-from-source compile failure instead of a clear "use
3.13" message.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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Co-authored-by: Jordan <jordanlgibbs@gmail.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>