Improve admin UI visual design — polish look and feel (#59)

* Improve admin UI visual design — polish look and feel

- Sticky nav bar with subtle shadow, branded GroomBook wordmark, green gradient Book button
- Consistent brand green (#4f8a6f) for primary buttons across all admin pages
- Tables wrapped in white cards with rounded corners and soft shadows
- Uppercase table headers with better spacing and hierarchy
- Input/button border-radius increased to 6px for softer feel
- Global CSS: button transitions, input focus states with brand green ring, subtle card shadows
- Background changed from plain white to light gray (#f0f2f5) for depth
- Reports: polished stat cards with shadows, refined section headers, card-wrapped tables
- Custom scrollbar styling for a cleaner look

Closes groombook/groombook#58

Co-Authored-By: Paperclip <noreply@paperclip.ing>

* Fix test selectors for branded nav text

- Use regex /Groom\s*Book/ to match split-element brand text
- Use getByRole("link") for Book CTA to avoid matching brand <strong>

Co-Authored-By: Paperclip <noreply@paperclip.ing>

* Fix brand text test to handle split-element rendering

The nav brand was changed to <span>Groom</span>Book for color styling,
but getByText with a regex can't match text split across child elements.
Use a custom text matcher that checks the STRONG element's textContent.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

* Fix E2E tests for split-element brand name

The brand is now <span>Groom</span>Book (no space), so Playwright's
getByText needs "GroomBook" instead of "Groom Book".

Co-Authored-By: Paperclip <noreply@paperclip.ing>

---------

Co-authored-by: Groom Book CTO <cto@groombook.dev>
Co-authored-by: Paperclip <noreply@paperclip.ing>
Co-authored-by: Groom Book CTO <cto@groombook.app>

apps/web/src/__tests__/App.test.tsx

@@ -23,12 +23,14 @@ function renderApp(route = "/admin") {
 describe("App navigation", () => {
   it("renders the Groom Book brand", () => {
     const nav = renderApp();
-    expect(within(nav).getByText("Groom Book")).toBeInTheDocument();
+    expect(
+      within(nav).getByText((_, el) => el?.tagName === "STRONG" && /Groom\s*Book/.test(el.textContent ?? ""))
+    ).toBeInTheDocument();
   });
 
   it("renders the Book CTA button", () => {
     const nav = renderApp();
-    expect(within(nav).getByText("Book")).toBeInTheDocument();
+    expect(within(nav).getByRole("link", { name: "Book" })).toBeInTheDocument();
   });
 
   it("renders all primary nav links", () => {
@@ -61,6 +63,8 @@ describe("App navigation", () => {
       </MemoryRouter>
     );
     // Customer portal should render at root - no admin nav present
-    expect(screen.queryByText("Groom Book")).not.toBeInTheDocument();
+    expect(
+      screen.queryByText((_, el) => el?.tagName === "STRONG" && /Groom\s*Book/.test(el.textContent ?? ""))
+    ).not.toBeInTheDocument();
   });
 });