fix: images in the Mac app; docs: a screenshot tour of every feature

The Mac app's content rules blocked the blob: URLs that the image viewer and
Markdown previews draw images from, so every image failed with "could not be
decoded". One anchored allow rule for blob: on the window's own origin fixes
it. Also: .jfif/.jpe/.apng/TIFF/HEIC open as images, an undecodable image
falls back to the hex dump, Markdown previews skip images over 20 MB
(/api/raw?max=), and the SVG source toggle ignores letter case.

The landing page gains "A tour of every feature": 20 features with light and
dark screenshots taken by `bun run shots` in a deterministic demo repository
(scripts/shots.ts, demo-repo.ts, packaging/macos/appshot.swift). Reviewed for
accuracy, visual design/accessibility and security (DESIGN.md round 10).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: jkyberneees