refactor(ui): extract NotFoundPage into its own i18n-aware module

Move the inline 404 component and its styles out of App.tsx into a
dedicated NotFoundPage module, and localize its copy via i18n keys.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/ui/pages/NotFoundPage/index.css

@@ -0,0 +1,83 @@
+.not-found-shell {
+  min-height: 100vh;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: 48px 24px;
+  color: #101827;
+  background: #fff;
+}
+
+.not-found-content {
+  width: min(520px, 100%);
+  display: grid;
+  justify-items: center;
+  text-align: center;
+}
+
+.not-found-mark {
+  width: min(280px, 72vw);
+  height: auto;
+  margin-bottom: 57px;
+}
+
+.not-found-content h1 {
+  margin: 0;
+  color: #111827;
+  font-size: 32px;
+  line-height: 1.25;
+  font-weight: 800;
+  letter-spacing: 0;
+}
+
+.not-found-content p {
+  margin: 16px 0 32px;
+  color: #6b7280;
+  font-size: 14px;
+  line-height: 1.5;
+  font-weight: 500;
+}
+
+.not-found-content button {
+  min-width: 128px;
+  height: 42px;
+  padding: 0 26px;
+  border-radius: 12px;
+  color: #fff;
+  background: #030303;
+  font-size: 16px;
+  font-weight: 700;
+  line-height: 1;
+  transition: transform 0.16s ease, background 0.16s ease;
+}
+
+.not-found-content button:hover {
+  background: #171717;
+  transform: translateY(-1px);
+}
+
+@media (max-width: 760px) {
+  .not-found-shell {
+    padding: 44px 22px;
+  }
+
+  .not-found-mark {
+    margin-bottom: 44px;
+  }
+
+  .not-found-content h1 {
+    font-size: 28px;
+  }
+
+  .not-found-content p {
+    margin: 14px 0 28px;
+    font-size: 14px;
+  }
+
+  .not-found-content button {
+    min-width: 120px;
+    height: 40px;
+    border-radius: 12px;
+    font-size: 15px;
+  }
+}