Fix #38: links open inside the BT standalone window on Vivaldi

[bennyz327]

Problem

Reported in #38 (Vivaldi 7.4): when the Topic Manager is run in WINDOW
(standalone panel) mode, clicking any link in the tree opens the
document inside the BT panel window, replacing the BT UI — rather
than in the user's main browser window. Only Vivaldi is affected;
Chrome, Edge behave correctly.

Root cause

background.js's openTabs / openTabGroups call chrome.tabs.create({url})
without a windowId whenever the parent topic has no siblings already open.
Chrome's currentWindow resolution explicitly skips popup / panel /
devtools window types, so on standard Chromium the new tab lands in
the user's most-recently-focused normal window — exactly what we want.

Vivaldi's currentWindow resolution does not exclude its own panel
windows, so the new tab is routed into the BT panel itself.

Fix

A small helper nonBTWindowId() returns the id of a non-BT normal
window to pass to chrome.tabs.create. It is a Vivaldi-only
workaround — it returns null on every other browser, so their code
paths are byte-identical to before this PR.

Vivaldi detection uses the vivExtData property that Vivaldi attaches
to every Window object (verified absent on Chrome/Edge). This is
robust to Vivaldi's UA-masking setting, which by default makes Vivaldi
indistinguishable from Chrome via navigator.userAgent /
userAgentData.

The helper is also used in openTabGroups so "Open All" / tab-group
restores get the same treatment.

Testing

• Vivaldi 7.9.3970.64 (Windows 11): WINDOW mode — single-link click and
  "Open All" both open in the user's main browser window; BT panel
  untouched. Sibling-grouping still routes to the sibling's window.
• Chrome stable: WINDOW / TAB / SIDEPANEL modes — behavior
  unchanged from before this PR (helper returns null, same code path
  as today).
• TAB mode and SIDEPANEL mode also unchanged on Vivaldi (helper early-returns
  before the Vivaldi probe).