refactor(frontend): simplify theme system and unify UI components

Remove the "casaos" theme in favor of a unified design system. This involves cleaning up conditional styling across components, simplifying the theme toggle, and updating the global CSS variables to a more consistent dark/light mode implementation.

- Remove `casaos` theme from `themes.ts` and `ThemeToggle`
- Refactor `globals.css` to use a single dark mode definition
- Simplify component styling by removing `isCasaOS` conditional logic
- Update UI components (`Card`, `Badge`, `WidgetCard`, etc.) to use standard design tokens
- Update E2E smoke tests to reflect theme changes

frontend/e2e/smoke.spec.ts

@@ -10,9 +10,9 @@ test("smoke: theme toggle works", async ({ page }) => {
   await page.goto("http://localhost:3000");
   const toggle = page.getByLabel("Toggle theme");
   await toggle.click();
-  await page.getByText("CasaOS").click();
+  await page.getByText("Light").click();
   const theme = await page.evaluate(() => document.documentElement.getAttribute("data-theme"));
-  expect(theme).toBe("casaos");
+  expect(theme).toBe("light");
 });
 
 test("smoke: empty state shows add button", async ({ page }) => {