[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/__tests__/i18n.test.js

@@ -31,7 +31,7 @@ describe('DC-077: i18n system', () => {
     });
 
     it('falls back to English for unsupported language', () => {
-      expect(i18n.t('dashboard.title', 'zh')).toBe('Dashboard');
+      expect(i18n.t('dashboard.title', 'xx')).toBe('Dashboard');
     });
 
     it('falls back to key if not found in any language', () => {
@@ -58,8 +58,8 @@ describe('DC-077: i18n system', () => {
     });
 
     it('returns false for unsupported languages', () => {
-      expect(i18n.isSupported('zh')).toBe(false);
-      expect(i18n.isSupported('ja')).toBe(false);
+      expect(i18n.isSupported('xx')).toBe(false);
+      expect(i18n.isSupported('klingon')).toBe(false);
     });
   });
 
@@ -81,14 +81,61 @@ describe('DC-077: i18n system', () => {
     });
 
     it('defaults to English for unsupported languages', () => {
-      expect(i18n.detectLanguage('zh-CN,zh;q=0.9')).toBe('en');
-      expect(i18n.detectLanguage('ja-JP,ja;q=0.9')).toBe('en');
+      expect(i18n.detectLanguage('xx-XX,xx;q=0.9')).toBe('en');
+      expect(i18n.detectLanguage('klingon-KL,klingon;q=0.9')).toBe('en');
     });
 
     it('strips region codes before matching', () => {
       expect(i18n.detectLanguage('en-US,en;q=0.9')).toBe('en');
       expect(i18n.detectLanguage('de-AT,de;q=0.9')).toBe('de');
     });
+
+
+    it('respects equal q-values by order', () => {
+      expect(i18n.detectLanguage('en;q=0.5,de;q=0.5')).toBe('en');
+    });
+
+    it('excludes q=0 entries per RFC 7231', () => {
+      expect(i18n.detectLanguage('en;q=0,fr;q=0.9')).toBe('fr');
+    });
+
+    it('serves default language when all entries have q=0 (intentional fallback)', () => {
+      expect(i18n.detectLanguage('en;q=0,fr;q=0')).toBe('en');
+    });
+
+    it('handles malformed q-values gracefully', () => {
+      // 'abc' is not a valid q-value per RFC 7231 grammar, so it is treated as
+      // "no q-value specified" — per the HTTP spec the default weight is q=1.0.
+      expect(i18n.detectLanguage('en;q=abc,fr;q=0.9')).toBe('en');
+    });
+
+    it('accepts q=0 boundary (excludes entry)', () => {
+      expect(i18n.detectLanguage('en;q=0,fr;q=0.9')).toBe('fr');
+    });
+
+    it('accepts q=1 boundary', () => {
+      expect(i18n.detectLanguage('en;q=1,fr;q=0.9')).toBe('en');
+    });
+
+    it('accepts q=1.0', () => {
+      expect(i18n.detectLanguage('en;q=1.0,fr;q=0.9')).toBe('en');
+    });
+
+    it('accepts q=0.001 (lowest non-zero weight)', () => {
+      expect(i18n.detectLanguage('en;q=0.001,fr;q=0.9')).toBe('fr');
+    });
+
+    it('accepts q=0.999', () => {
+      expect(i18n.detectLanguage('en;q=0.999,fr;q=0.9')).toBe('en');
+    });
+
+    it('rejects q=1.001 (RFC invalid) — defaults to 1.0', () => {
+      expect(i18n.detectLanguage('en;q=1.001,fr;q=0.9')).toBe('en');
+    });
+
+    it('handles uppercase Q parameter', () => {
+      expect(i18n.detectLanguage('en;Q=0.5,fr;q=0.9')).toBe('fr');
+    });
   });
 
   describe('RTL support', () => {

dashcaddy-api/__tests__/routes/i18n-routes.test.js

@@ -14,13 +14,13 @@ function createI18nApp() {
 }
 
 describe('DC-077: i18n Routes', () => {
-  it('GET /i18n/languages returns 5 languages', async () => {
+  it('GET /i18n/languages returns 31 languages', async () => {
     const app = createI18nApp();
     const res = await request(app).get('/api/v1/i18n/languages');
 
     expect(res.status).toBe(200);
     expect(res.body.success).toBe(true);
-    expect(res.body.languages).toHaveLength(5);
+    expect(res.body.languages).toHaveLength(31);
     expect(res.body.default).toBe('en');
   });