fix(theme): truncate long nav title with ellipsis

[xbsheng]

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

[bluwy]

It looks like without min-width: 0 and white-space: nowrap, it also renders fine. Maybe we can remove it?

[xbsheng]

You're right, both are redundant — verified and removed in 236a879:

• min-width: 0: the span already has overflow: hidden, and per the flexbox spec an item with overflow != visible computes its automatic minimum size as 0, so the explicit declaration is a no-op.
• white-space: nowrap: .VPNavBar already sets white-space: nowrap and the span inherits it.

The remaining rule is just overflow: hidden; text-overflow: ellipsis. The e2e tests (both dev and build mode) still pass, and the rendered title stays on a single line within the nav.

[bluwy]

This seems good to me, but I don't know what's the convention for style-related tests. Personally I think it's fine without, but I'll leave @brc-dd to decide here.