fix: Login zentrieren und CSV-Unterschrift-Platzhalter übersetzen

Auth-Screens werden per auth-screen über die volle Viewport-Höhe zentriert.
Bild-Unterschriften im CSV-Export nutzen i18n statt festem deutschen Text.

Co-authored-by: Cursor <cursoragent@cursor.com>

client/src/App.css

@@ -4,12 +4,27 @@ body {
   background: radial-gradient(circle at center, #1b264f 0%, #0b0c10 100%);
   min-height: 100vh;
   margin: 0;
-  display: flex;
-  justify-content: center;
-  align-items: center;
   font-family: system-ui, -apple-system, sans-serif;
 }
 
+#root:has(.auth-screen) {
+  width: 100%;
+  max-width: none;
+  border-inline: none;
+  text-align: initial;
+}
+
+.auth-screen {
+  flex: 1;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 100%;
+  min-height: 100svh;
+  padding: 24px 16px calc(48px + env(safe-area-inset-bottom, 0px));
+  box-sizing: border-box;
+}
+
 /* Glassmorphism Auth Card */
 .auth-card {
   background: rgba(11, 12, 16, 0.75);