fix(theme): truncate long nav title with ellipsis - #5376

Problem

When the site title is longer than the sidebar width, the nav title text visually overflows its bordered title area and spills over the page content — e.g. 从零实现 DeepSeek Harness (~330px) vs the 17rem sidebar.

The title <span> is a flex item inside .VPNavBarTitle .title (display: flex). Flex items default to min-width: auto, so the span refuses to shrink below its text width and simply overflows the container (overflow: visible). The wrapper is already sized to var(--vp-sidebar-width) when a sidebar is present, so the only leak is the text itself.

Fix

Constrain the title text:

.title span {
  min-width: 0;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

• min-width: 0 lets the flex item shrink below its content width
• white-space: nowrap + overflow: hidden + text-overflow: ellipsis truncate the text instead of overflowing

No layout change when the title is short (no truncation kicks in), and non-sidebar pages are untouched (the wrapper there sizes to content).

Tests

Added __tests__/e2e/nav-title.test.ts with a deliberately long site title in the e2e fixture site:

• title text does not overflow the sidebar (bounding-box comparison)
• title text is truncated with ellipsis (scrollWidth > clientWidth)

Both fail without the fix, pass with it. Full suite green: test:types, test:unit (296), e2e dev (41) and e2e build (39) all pass.

Checklist

• pnpm test:types passes
• pnpm test:unit passes
• pnpm test:e2e (dev + build) passes