feat: responsive nav — hamburger menu + wider container (900px breakpoint)

- Widen nav container from 72rem to 80rem (1280px) — matches Zillow's
  nav container width, more breathing room for items on large monitors
- Raise collapse breakpoint from 768px to 899px — links stay visible
  until the screen is actually too narrow
- Add hamburger button (SVG 3-line icon) visible at < 900px
- Add mobile drop-down panel with all nav links grouped under Plan /
  Explore / Account sections; overlay + Escape key close it

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -6,6 +6,15 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ## [Unreleased]
 
+### Added
+- Nav: hamburger menu on screens < 900px — clicking opens a full-width mobile panel with all nav links; overlay click and Escape key close it
+- Nav: mobile panel groups links under "Plan", "Explore", and "Account" section headers
+
+### Changed
+- Nav: widen container from 72rem (1152px) to 80rem (1280px) — matches Zillow's nav container width, more breathing room for nav items on large monitors
+- Nav: collapse breakpoint raised from 768px to 899px — nav links no longer hide until the screen is actually too narrow
+- Nav: remove redundant inline `style="display:grid;grid-template-columns:1fr auto 1fr"` on `.nav-inner` (already in CSS)
+
 ### Fixed
 - i18n: improve German nav labels — "Verzeichnis" → "Anbieterverzeichnis", "Planer" → "Kostenrechner"
 - CI: add missing env vars to `.env` heredoc — `WAITLIST_MODE`, `LEADS_EMAIL`, `UMAMI_API_URL`; make Paddle vars optional (`:-`) so they don't break deploys when unset