Remove page subtitles from all dashboard pages

Subtitles added noise without value. Stripped subtitle props from all 8
PageHeader calls, removed subtitle display from TopBar, and simplified
the PageTitleContext and PageHeader interfaces to title-only.

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

src/lib/page-title-context.tsx

@@ -2,33 +2,25 @@
 
 import { createContext, useContext, useState, useCallback, ReactNode } from 'react';
 
-interface PageTitleState {
+interface PageTitleContextValue {
   title: string;
-  subtitle: string;
-}
-
-interface PageTitleContextValue extends PageTitleState {
-  setPageTitle: (title: string, subtitle: string) => void;
+  setPageTitle: (title: string) => void;
 }
 
 const PageTitleContext = createContext<PageTitleContextValue>({
   title: '',
-  subtitle: '',
   setPageTitle: () => {},
 });
 
 export function PageTitleProvider({ children }: { children: ReactNode }) {
-  const [state, setState] = useState<PageTitleState>({ title: '', subtitle: '' });
+  const [title, setTitle] = useState('');
 
-  const setPageTitle = useCallback((title: string, subtitle: string) => {
-    setState(prev => {
-      if (prev.title === title && prev.subtitle === subtitle) return prev;
-      return { title, subtitle };
-    });
+  const setPageTitle = useCallback((t: string) => {
+    setTitle(prev => prev === t ? prev : t);
   }, []);
 
   return (
-    <PageTitleContext.Provider value={{ ...state, setPageTitle }}>
+    <PageTitleContext.Provider value={{ title, setPageTitle }}>
       {children}
     </PageTitleContext.Provider>
   );