docs: fix sidebar overflow at mid-width viewports and add a favicon (#2426)

docs/.vitepress/theme/custom.css

@@ -13,9 +13,16 @@
  * which forces horizontal scrolling on most of our ~100-column code blocks.
  * Let the layout breathe and the column grow so typical snippets fit;
  * genuinely long lines still scroll inside their own block.
+ *
+ * Clamped to the viewport because the default theme's `(min-width: 1440px)`
+ * rules size the sidebar and nav title as
+ * `calc((100% - (var(--vp-layout-max-width) - 64px)) / 2 + ...)`, which goes
+ * negative on viewports narrower than the max width: the sidebar collapses
+ * and the site title spills over the search bar. With the clamp, those rules
+ * resolve to their stock sub-1440px values on narrower viewports instead.
  */
 :root {
-    --vp-layout-max-width: 1680px;
+    --vp-layout-max-width: min(100vw, 1680px);
 }
 
 /* !important: the default rule is a scoped component style ([data-v-…]), which