feat(client): add Timestamp component, ESLint 10 + Prettier, and code quality fixes

- add Timestamp component with moment relative time and ISO tooltip
- add Timestamp stories (JustNow, MinutesAgo, HoursAgo, DaysAgo, MonthsAgo)
- add ESLint 10 with typescript-eslint, react-hooks, react-refresh, prettier
- add Prettier config with singleQuote, semi, trailingComma all, printWidth 100
- add lint, lint:fix, format, test:storybook scripts to package.json
- fix react-hooks/set-state-in-effect in all page components
- auto-fix 113 Prettier formatting issues across src and .storybook

client/src/hooks/useTheme.ts

@@ -19,7 +19,7 @@ export function useTheme() {
   }, [theme]);
 
   function toggleTheme() {
-    setTheme(t => (t === 'light' ? 'dark' : 'light'));
+    setTheme((t) => (t === 'light' ? 'dark' : 'light'));
   }
 
   return { theme, toggleTheme } as const;