fix: sidebar CSS overflow for long titles (#170)

[giwaov]

Summary

Fixes #170 long titles in the sidebar (e.g. Cross-Component Calls, Toolchain & Project Structure, Patterns & Security) were overflowing the sidebar container horizontally.

Changes

src/custom/_sidebar.css:

• overflow-x: hidden on the sidebar container and menu element
• .menu__link uses word-wrap, overflow-wrap, and hyphens for graceful wrapping
• code elements inside sidebar links use word-break: break-all to prevent overflow
• Category labels (collapsible headers) truncate with ext-overflow: ellipsis on a single line
• Leaf item links (actual page links) wrap naturally across multiple lines for readability

Test Plan

• Build passes with
  pm run build
• Sidebar renders correctly with long section titles at standard viewport widths
• No regression on short titles or mobile navbar