VitePress reserves navbar title space only when a sidebar exists

At desktop widths, pages configured with `sidebar: false` and 404 pages were still reserving a sidebar-sized title column in the navbar, which pushed the search button away from the title even when no sidebar was rendered [1]. The theme applied the title's minimum width unconditionally; it now applies that minimum width only when `.has-sidebar` is present, alongside the existing maximum width, so sidebar alignment is unchanged while sidebar-free pages get the compact layout back [1][2].

The change adds coverage for home, sidebar-free, 404, and sidebar pages [2], including a browser regression test comparing title-to-search spacing, so the conditional behavior is exercised across the four page types rather than asserted statically [1].

No public API, config key, or version number changed: this is a scoped theme style fix. If your docs site sets `sidebar: false` on any page, the layout change lands with the next VitePress release.