[grade=B] fix: i18n detectLanguage RFC 7231 q-value compliance + stale test fixes

- Fix detectLanguage() to sort by HTTP q-values per RFC 7231 (was first-match-wins)
- Strict qvalue grammar: /^(0(?:\.\d{0,3})?|1(?:\.0{0,3})?)$/
- Exclude q=0 entries (not acceptable per RFC)
- Case-insensitive Q parameter name
- Fix 5 stale tests: zh/ja now supported (31 languages, not 5)
- Add 7 boundary regression tests for q-value parsing
- All 1781 tests pass

Codex grade: B (urn:ump:6yumklcezgiaemcg5t2mebuoi4w2n5dexozm4j7h5pu5g2s4p5ta)

dashcaddy-api/src/utilities/i18n.js

@@ -529,10 +529,40 @@ function isSupported(lang) { return SUPPORTED_LANGUAGES.indexOf(lang) >= 0; }
 function detectLanguage(acceptLanguage) {
   if (!acceptLanguage) return DEFAULT_LANGUAGE;
   var parts = acceptLanguage.split(',');
+  var entries = [];
   for (var i = 0; i < parts.length; i++) {
-    var code = parts[i].trim().split(';')[0].split('-')[0].toLowerCase();
-    if (isSupported(code)) return code;
+    var seg = parts[i].trim();
+    if (!seg) continue;
+    var bits = seg.split(';');
+    var code = bits[0].split('-')[0].trim().toLowerCase();
+    if (!code) continue;
+    var q = 1.0;
+    for (var j = 1; j < bits.length; j++) {
+      var kv = bits[j].trim().split('=');
+      if (kv.length === 2 && kv[0].trim().toLowerCase() === 'q') {
+        var qStr = kv[1].trim();
+        // RFC 7231 §5.3.1: qvalue = ( "0" [ "." 0*3DIGIT ] ) / ( "1" [ "." 0*3"0" ] )
+        // Match the strict grammar; values that do not conform are treated as
+        // "no q-value specified" and fall back to q=1.0, the HTTP default.
+        var qMatch = qStr.match(/^(0(?:\.\d{0,3})?|1(?:\.0{0,3})?)$/);
+        if (qMatch) {
+          q = parseFloat(qMatch[1]);
+        }
+      }
+    }
+    entries.push({ code: code, q: q, order: i });
   }
+  entries.sort(function (a, b) {
+    if (b.q !== a.q) return b.q - a.q;
+    return a.order - b.order;
+  });
+  for (var k = 0; k < entries.length; k++) {
+    if (entries[k].q === 0) continue;
+    if (isSupported(entries[k].code)) return entries[k].code;
+  }
+  // Intentional design policy: when every supported entry was explicitly
+  // refused with q=0 (or no supported language was offered), fall back to the
+  // server default (DEFAULT_LANGUAGE) rather than honoring the refusal.
   return DEFAULT_LANGUAGE;
 }