feat(09-06): MergeTab multi-file UI + PFX convert option + render tests

- actions.ts: mergeCertsAction(files, outputFormat, password?) builds FormData with
  multiple file fields; delegates to postForm('merge', ...) (T-09-02/T-09-04)
- MergeTab.tsx: multi-file state (local), file input (multiple), output selector
  (pem|pfx), Zusammenfuehren button disabled when < 2 files (data-testid for tests),
  onOutputFormatChange callback to page.tsx for shared PasswordField visibility
- ConvertTab.tsx: gains pfx option + onTargetFormatChange callback (same pattern)
- page.tsx: lifts mergeOutputFormat + convertOutputFormat state; showPassword now
  also true when active tab's output format is 'pfx'; passes callbacks to tabs
- cert-manager.test.tsx: 5 new tests — MergeTab disabled/enabled by file count,
  shared PasswordField appears on pfx output, downloadBase64 called on success;
  ConvertTab pfx option present; all 19/19 web tests green
- All production cert-manager files type-clean (pre-existing test type issues unchanged)

apps/web/src/app/(portal)/modules/cert-manager/page.tsx

@@ -25,6 +25,10 @@ function isPfxFile(file: File | null): boolean {
  * Layout per UI-SPEC: max-w-4xl, shared input card, tab nav, tab content card.
  * T-09-02: password lives in local React state only; never logged or placed in URLs.
  * T-09-04: all API calls via postForm() which sends credentials:'include'.
+ *
+ * The shared PasswordField is shown when:
+ * 1. A PFX/P12 file is selected in the shared DropZone, OR
+ * 2. The active tab (Merge or Convert) has PFX chosen as the output format.
  */
 export default function CertManagerPage() {
   const t = useTranslations('certManager');
@@ -34,7 +38,15 @@ export default function CertManagerPage() {
   const [pemText, setPemText] = useState('');
   const [password, setPassword] = useState('');
 
-  const showPassword = isPfxFile(file);
+  // Track the output format of the active Merge and Convert tabs
+  // so the shared PasswordField can be shown when PFX output is chosen.
+  const [mergeOutputFormat, setMergeOutputFormat] = useState('pem');
+  const [convertOutputFormat, setConvertOutputFormat] = useState('pem');
+
+  const showPassword =
+    isPfxFile(file) ||
+    (activeTab === 'merge' && mergeOutputFormat === 'pfx') ||
+    (activeTab === 'convert' && convertOutputFormat === 'pfx');
 
   const handleFile = (selected: File) => {
     setFile(selected);
@@ -61,16 +73,27 @@ export default function CertManagerPage() {
   };
 
   const renderActiveTab = () => {
-    const props = { file, pemText, password };
     switch (activeTab) {
       case 'inspect':
-        return <InspectTab {...props} />;
+        return <InspectTab file={file} pemText={pemText} password={password} />;
       case 'split':
-        return <SplitTab {...props} />;
+        return <SplitTab file={file} pemText={pemText} password={password} />;
       case 'merge':
-        return <MergeTab {...props} />;
+        return (
+          <MergeTab
+            password={password}
+            onOutputFormatChange={setMergeOutputFormat}
+          />
+        );
       case 'convert':
-        return <ConvertTab {...props} />;
+        return (
+          <ConvertTab
+            file={file}
+            pemText={pemText}
+            password={password}
+            onTargetFormatChange={setConvertOutputFormat}
+          />
+        );
     }
   };