Fix: localize reply/forward quote header incl. sender address

The reply/forward quote header was always emitted in English ("On {date},
{from} wrote:", "---------- Forwarded message ----------", From/Date/Subject)
regardless of UI language, in both the main path (lib/quote-header.ts) and the
composer's inline fallback. quote-header.ts now takes an optional localized
QuoteHeaderLabels set (English defaults preserved for back-compat); page.tsx
builds it from a new quote_header message namespace, and the composer fallback
uses the same keys. Added the quote_header namespace to all 17 locales.

Also folds in the forward-sender-address fix: the forward "From:" line now
shows the full "Name <email>" like every mail client (the reply line keeps the
bare name, which reads naturally in "On … wrote:").

locales/de/common.json

@@ -1,4 +1,11 @@
 {
+  "quote_header": {
+    "reply_line": "Am {date} schrieb {from}:",
+    "forwarded_separator": "---------- Weitergeleitete Nachricht ----------",
+    "from_label": "Von",
+    "date_label": "Datum",
+    "subject_label": "Betreff"
+  },
   "login": {
     "title": "Webmail",
     "username_label": "E-Mail",