Simplify per Alanas

Simplify does two things:

Reader Mode — strips clutter from articles and shows them in a clean,
themeable view. Fully local.

AI Simplify Selection — sends selected text to our AI for plain-language
rewriting. Off by default. First use shows a consent prompt; we store the
"yes" flag locally so we don't ask again.

Test:
1. Open https://en.wikipedia.org/wiki/Mars.
2. Press Alt+Shift+S — reader mode opens. Try the theme switcher. Escape
to exit.
3. Select any paragraph, right-click → "Simplify Selection". Accept the
prompt. The text comes back simpler. With the AI service offline, a
local heuristic kicks in silently.

Permissions:
- activeTab + scripting: inject the reader and selection UI.
- contextMenus: the right-click action.
- storage: themes + the AI consent flag.
- host_permissions for our Railway endpoint: only hit after opt-in, only
for the AI feature.

Vanilla JS. No bundler. No minification. The zip is the source.