refactor: Context-based theme provider with animated Typewriter toggle

- Convert theme to SolidJS Context/Provider pattern (ThemeProvider)
- Extract createThemeState() for testability without context
- Add Typewriter component with character-by-character reveal
- Animate ThemeToggle with Typewriter label and hover scale
- Add cursor CSS animations (typewriter-blink, cursor-typing, cursor-block)
- Fix background color transition by using 'all' on :root
- Rename theme.ts -> theme.tsx for JSX support
- All 26 theme tests passing

web/src/app.tsx

@@ -2,13 +2,13 @@ import { MetaProvider, Title } from "@solidjs/meta";
 import { Router } from "@solidjs/router";
 import { FileRoutes } from "@solidjs/start/router";
 import { Suspense } from "solid-js";
-import { useTheme } from "./lib/theme";
+import { ThemeProvider } from "./lib/theme";
 import { AppShell } from "./components/layout";
 import "./app.css";
 
 export default function App() {
-  useTheme();
   return (
+    <ThemeProvider>
     <Router
       root={(props) => (
         <AppShell>
@@ -18,5 +18,6 @@ export default function App() {
     >
       <FileRoutes />
     </Router>
+    </ThemeProvider>
   );
 }