fix(webui): keep search rail controls inside the sidebar

[balcsida]

Summary

On the search page, the repository picker, the language filter and the example buttons extended past the sidebar's right border. The sidebar is a single-column grid with an implicit auto track. That track grew to fit the widest content, the unwrapped example case:yes repo:payments Token (about 217px), but the sidebar only has 200px inside its padding.

The fix sets grid-template-columns:minmax(0,1fr) on .search-rail, so the column can't grow wider than the sidebar. The example buttons already had text-overflow:ellipsis, so the long example is now truncated as intended, and clicking it still fills in the full query.

Verification

• Rendered the search view in headless Chromium at 1728px wide.
  • Before: the sidebar ends at x=396, and every control ends at x=397, past the border.
  • After: every control ends at x=379, leaving the 16px padding before the divider.
• go test ./internal/webui/...: 88 passed.

There are no configuration, schema or security changes. The only file changed besides the changelog is internal/webui/index.html (CSS).

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