feat(pitch-deck): change preferred_lang for existing investors from detail page

- GET /api/admin/investors/:id now returns preferred_lang
- PATCH /api/admin/investors/:id accepts preferred_lang (de/en), validates value
- Investor detail page: DE/EN toggle in the Pitch Version card, instant save on click

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pitch-deck/app/pitch-admin/(authed)/investors/[id]/page.tsx

@@ -22,6 +22,7 @@ interface InvestorDetail {
     version_name: string | null
     version_status: string | null
     is_showcase: boolean
+    preferred_lang: 'de' | 'en'
   }
   sessions: Array<{
     id: string
@@ -324,6 +325,39 @@ export default function InvestorDetailPage() {
           </span>
         </div>
 
+        {/* Default language */}
+        <div className="mt-4 pt-4 border-t border-white/[0.06]">
+          <div className="flex items-center justify-between">
+            <div>
+              <div className="text-sm text-white font-medium">Default pitch language</div>
+              <div className="text-xs text-white/40 mt-0.5">Language the deck opens in when the investor clicks the link</div>
+            </div>
+            <div className="flex rounded-lg border border-white/10 overflow-hidden">
+              {(['de', 'en'] as const).map(l => (
+                <button
+                  key={l}
+                  disabled={busy}
+                  onClick={async () => {
+                    if (inv.preferred_lang === l) return
+                    setBusy(true)
+                    const res = await fetch(`/api/admin/investors/${id}`, {
+                      method: 'PATCH',
+                      headers: { 'Content-Type': 'application/json' },
+                      body: JSON.stringify({ preferred_lang: l }),
+                    })
+                    setBusy(false)
+                    if (res.ok) { flashToast(`Language set to ${l.toUpperCase()}`); load() }
+                    else { flashToast('Update failed') }
+                  }}
+                  className={`px-4 py-1.5 text-sm font-medium transition-colors disabled:opacity-50 ${inv.preferred_lang === l ? 'bg-indigo-600 text-white' : 'bg-black/20 text-white/50 hover:text-white/80'}`}
+                >
+                  {l === 'de' ? '🇩🇪 DE' : '🇬🇧 EN'}
+                </button>
+              ))}
+            </div>
+          </div>
+        </div>
+
         {/* Showcase toggle */}
         <div className="flex items-center justify-between mt-4 pt-4 border-t border-white/[0.06]">
           <div>