fix: dock the editor in the page in Vivaldi

[ankit]

Fixes #1184. Vivaldi ignores per-tab side panels and shares one panel across every tab (bug VB-120826), so with the side panel chosen the editor was blank or attached to the wrong tab. Vivaldi can't be detected by user agent, so this reacts to how its panel behaves and docks the editor in the page instead.

• When the editor's panel page loads hosted as a tab, which Vivaldi does and Chrome never does, it saves a flag and closes the panel.
• After opening the side panel, the background waits up to 2 seconds for that flag, then switches the position to "dock right in page" and opens the editor there.
• With the flag set, the side panel is no longer offered or used.

Review notes: no manifest change; Chrome, Edge and Firefox behave as before, and only the panel's own report can switch the side panel off. The shortcut can still flash Vivaldi's panel once after Stylebot's background has gone idle, since the panel must open before the flag is read. Earlier commits tried other approaches, so please squash-merge.

Testing: unit tests for the check; side panel e2e tests pass, including one showing a normal Chrome panel stays the chosen position. Tested by hand in Vivaldi.

🤖 Generated with Claude Code

[github-actions]

Bundle size

This PR changes what the popup or content scripts load.

Bundle	Size	Change	Gzip	Change
Popup	174.7 KB	+1.9 KB (1.1%)	48.1 KB	+109 B (0.2%)
fonts/fonts.css	2.9 KB	–	458 B	–
popup/index.css	23.4 KB	–	5.2 KB	–
popup/index.js	148.4 KB	+1.9 KB (1.3%)	42.5 KB	+109 B (0.3%)
Content script (document_start)	78.3 KB	–	21.3 KB	–
editor/index.js	40.4 KB	–	10.3 KB	–
inject-css/index.js	34.8 KB	–	9.6 KB	–
site-bridge/index.js	3.1 KB	–	1.4 KB	–