fix: Lumière layout/navbar on settings+404, drop proxy_request_buffering

- Move min-height:100vh from body to html:has(body[data-lumiere]) so the
  gradient fills the viewport on short pages without inflating document
  height or pushing the footer below the fold
- Remove min-height:60vh from .error-container (was causing scrollbar on
  404 when combined with thin footer)
- Replace Bootstrap 3 navbar selectors (.navbar-nav > li > a) with CSS
  custom property overrides (--navbar-link-color, --navbar-link-hover-*,
  --navbar-bg, etc.) consumed by navbar.scss — fixes header button colours
- Remove position:relative from .navbar-default override; base CSS already
  has position:absolute which provides the stacking context for ::before
- Drop proxy_request_buffering off from upload location: buffered mode +
  global client_body_timeout 15m (nginx.conf.template) is more compatible
  with multer's multipart stream handling on slow connections

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

server-ce/nginx/overleaf.conf

@@ -9,8 +9,9 @@ server {
 		internal;
 	}
 
-	# File upload endpoints: stream directly to Node.js with generous timeouts
-	# for large files on slow / unreliable connections.
+	# File upload endpoints: extended timeouts for large files on slow connections.
+	# client_body_timeout 15m is set globally in nginx.conf.template; repeated
+	# here explicitly and client_max_body_size raised to 550m for this path.
 	location ~ ^/project/[^/]+/upload$ {
 		proxy_pass http://127.0.0.1:4000;
 		proxy_http_version 1.1;
@@ -21,7 +22,6 @@ server {
 		proxy_send_timeout 15m;
 		client_body_timeout 15m;
 		client_max_body_size 550m;
-		proxy_request_buffering off;
 	}
 
 	location / {