feat(ui): add public landing page with impressum and privacy pages

Introduce a marketing landing page at `/` with hero section, feature grid,
how-it-works steps, CTA banner, and footer. Move the authenticated dashboard
to `/dashboard`. Add static Impressum and Privacy Policy pages for EU legal
compliance. Update login redirect defaults accordingly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/login.rs

@@ -1,11 +1,26 @@
 use crate::Route;
 use dioxus::prelude::*;
+
+/// Login redirect component.
+///
+/// Redirects the user to the external OAuth authentication endpoint.
+/// If no `redirect_url` is provided, defaults to `/dashboard`.
+///
+/// # Arguments
+///
+/// * `redirect_url` - URL to redirect to after successful authentication
 #[component]
 pub fn Login(redirect_url: String) -> Element {
     let navigator = use_navigator();
 
     use_effect(move || {
-        let target = format!("/auth?redirect_url={}", redirect_url);
+        // Default to /dashboard when redirect_url is empty.
+        let destination = if redirect_url.is_empty() {
+            "/dashboard".to_string()
+        } else {
+            redirect_url.clone()
+        };
+        let target = format!("/auth?redirect_url={destination}");
         navigator.push(NavigationTarget::<Route>::External(target));
     });