[ankit]

Fixes #1210. Arc has the side panel and commands APIs but doesn't honour them: it loads the side panel without showing it, and never passes Stylebot's shortcuts on.

• Detect Arc by its --arc-palette-* CSS variables (its user agent matches Chrome's); the editor then docks in the page, as in Vivaldi.
• In Arc, the content script catches Stylebot's shortcut keys and has the background run them.
• The toggle-editor shortcut only tries the side panel when it's the chosen dock.
• Vivaldi and Arc quirks live in one chromium-variants module; Vivaldi's old stored flag still reads.

Review notes: in Arc, shortcuts only work while a page has focus.

Testing: unit, Storybook and side panel/shortcut/popup e2e on Chrome; checked by hand in Arc.

🤖 Generated with Claude Code

[github-actions]

Bundle size

This PR changes what the popup or content scripts load.

Bundle	Size	Change	Gzip	Change
Popup	178.5 KB	+1.5 KB (0.8%)	48.8 KB	+366 B (0.7%)
fonts/fonts.css	2.9 KB	–	458 B	–
popup/index.css	23.3 KB	–	5.2 KB	–
popup/index.js	152.3 KB	+1.5 KB (1.0%)	43.1 KB	+366 B (0.8%)
Content script (document_start)	89.4 KB	+9.1 KB (11.4%)	23.0 KB	+1.4 KB (6.6%)
editor/index.js	50.2 KB	+9.1 KB (22.2%)	11.8 KB	+1.4 KB (13.7%)
inject-css/index.js	36.1 KB	–	9.8 KB	–
site-bridge/index.js	3.1 KB	–	1.4 KB	–