chore: rename MedAssist to MedAssist-ng in all frontend UI (#74)

Update all visible text from 'MedAssist' to 'MedAssist-ng':
- Auth page titles (login, register)
- Loading/error/initializing states
- SharedSchedule page (loading, expired, error, footer)
- AboutModal fallback text
- i18n strings for export file validation (EN/DE)
- Related test expectations

frontend/src/test/components/SharedSchedule.test.tsx

@@ -1369,7 +1369,7 @@ describe("SharedSchedule footer and branding", () => {
 
 		const link = footer?.querySelector('a[href="/"]');
 		expect(link).toBeInTheDocument();
-		expect(link?.textContent).toBe("MedAssist");
+		expect(link?.textContent).toBe("MedAssist-ng");
 	});
 
 	it("displays sharedBy username in footer", async () => {