fix(single-tab): preserve window controls and New Tab

[adamXbot]

Selecting singletabhidden.css hid the entire #TabsToolbar with one tab open. Its window controls and New Tab buttons inherited visibility: collapse, causing the missing buttons reported in #227.

Hide only the single-tab strip and let the toolbar size itself around its remaining widgets. Window controls stay available, and Firefox's standalone New Tab button replaces the copy inside the hidden strip. Moving New Tab into the address toolbar removes the empty row entirely; keeping it in the tab toolbar retains a row for that button. The README now explains both placements.

The option stays inactive for pinned tabs, groups, vertical tabs and toolbar customization. Default fxcss installs still omit this optional sheet.

Validation:

• Firefox 156.0.1: regression checks passed locally on macOS and in the Windows/Linux run.
• Checks cover the GitHub Dark palette, default/optional fxcss installation, opening a second tab and returning to one, window controls, New Tab visibility/hit testing and its command, both window-button placements, compact/customized toolbars, pinned/grouped/vertical tabs and customization. Windows also checks the maximized window.
• The same check fails against unmodified master because the control container inherits visibility: collapse.
• Workflow lint and whitespace checks passed.

Fixes #227.

[github-actions]

Theme preview

Rendered from 7c4154f against the base branch.

Linux — no visual change

Nothing rendered differently. Current appearance:

macOS — no visual change

Nothing rendered differently. Current appearance:

Windows — no visual change

Nothing rendered differently. Current appearance:

No pixel differences on any platform — this PR does not change how the browser chrome renders in these views.

_{Views: browser window, focused address bar, find bar, modal dialog — each in light and dark. Native popups (app menu, context menus) are OS-level windows and cannot be captured. Workflow run}