refactor: remove unnecessary useMemo calls across components

Replace over-cached simple computations with direct expressions:

- popup/App.tsx: getEntryPointType() and routerConfig (values are stable)
- Dashboard/index.tsx: visibleSet, visibleFeatures, recentFeatures
  (Set creation + array filter/map are cheaper than useMemo overhead)
- useTextStatistics.ts: getTextStats(text) (simple string processing)
- ThemeModeProvider.tsx: contextValue object (setMode is stable via useCallback)

-39 lines, +28 lines (net -11 lines)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/providers/ThemeModeProvider.tsx

@@ -1,12 +1,4 @@
-import {
-  createContext,
-  ReactNode,
-  useCallback,
-  useContext,
-  useEffect,
-  useMemo,
-  useState,
-} from 'react';
+import { createContext, ReactNode, useCallback, useContext, useEffect, useState } from 'react';
 import { browser } from 'wxt/browser';
 import { storageUtil } from '@/utils/chromeStorage';
 
@@ -141,12 +133,11 @@ export function ThemeModeProvider({ children }: ThemeModeProviderProps) {
     }
   }, [resolvedMode]);
 
-  const contextValue = useMemo(
-    () => ({ mode, resolvedMode, setMode }),
-    [mode, resolvedMode, setMode],
+  return (
+    <ThemeModeContext.Provider value={{ mode, resolvedMode, setMode }}>
+      {children}
+    </ThemeModeContext.Provider>
   );
-
-  return <ThemeModeContext.Provider value={contextValue}>{children}</ThemeModeContext.Provider>;
 }
 
 export function useThemeMode(): ThemeModeContextType {