feat: add mobile-friendly HTTP 404 and 500 error pages

Add ErrorNotFound and ErrorInternal handler methods that render styled
error pages using the error.html template, with proper status codes,
responsive layout, SVG icons, and HTMX fragment support. Replace the
plain-text http.NotFound call in Dashboard with the new styled handler.

Closes leeworks-agents/gitea-mobile#131

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/handlers/handlers_test.go

@@ -183,6 +183,87 @@ func TestAddComment_EmptyBody(t *testing.T) {
 	}
 }
 
+func TestErrorNotFound(t *testing.T) {
+	h := newTestHandler()
+	req := httptest.NewRequest(http.MethodGet, "/nonexistent", nil)
+	w := httptest.NewRecorder()
+
+	h.ErrorNotFound(w, req)
+
+	if w.Code != http.StatusNotFound {
+		t.Errorf("status = %d, want %d", w.Code, http.StatusNotFound)
+	}
+	body := w.Body.String()
+	if body == "" {
+		t.Error("expected non-empty response body")
+	}
+	if !contains(body, "404") {
+		t.Error("expected body to contain '404'")
+	}
+	if !contains(body, "Page Not Found") {
+		t.Error("expected body to contain 'Page Not Found'")
+	}
+}
+
+func TestErrorInternal(t *testing.T) {
+	h := newTestHandler()
+	req := httptest.NewRequest(http.MethodGet, "/error", nil)
+	w := httptest.NewRecorder()
+
+	h.ErrorInternal(w, req)
+
+	if w.Code != http.StatusInternalServerError {
+		t.Errorf("status = %d, want %d", w.Code, http.StatusInternalServerError)
+	}
+	body := w.Body.String()
+	if body == "" {
+		t.Error("expected non-empty response body")
+	}
+	if !contains(body, "500") {
+		t.Error("expected body to contain '500'")
+	}
+	if !contains(body, "Internal Server Error") {
+		t.Error("expected body to contain 'Internal Server Error'")
+	}
+}
+
+func TestDashboard_NonRootPath_Returns404(t *testing.T) {
+	h := newTestHandler()
+	req := httptest.NewRequest(http.MethodGet, "/unknown/path", nil)
+	w := httptest.NewRecorder()
+
+	h.Dashboard(w, req)
+
+	if w.Code != http.StatusNotFound {
+		t.Errorf("status = %d, want %d", w.Code, http.StatusNotFound)
+	}
+	body := w.Body.String()
+	if !contains(body, "404") {
+		t.Error("expected body to contain '404' for non-root path")
+	}
+}
+
+func TestErrorNotFound_HTMX(t *testing.T) {
+	h := newTestHandler()
+	req := httptest.NewRequest(http.MethodGet, "/nonexistent", nil)
+	req.Header.Set("HX-Request", "true")
+	w := httptest.NewRecorder()
+
+	h.ErrorNotFound(w, req)
+
+	if w.Code != http.StatusNotFound {
+		t.Errorf("status = %d, want %d", w.Code, http.StatusNotFound)
+	}
+	body := w.Body.String()
+	// HTMX response should not contain DOCTYPE.
+	if contains(body, "<!DOCTYPE") {
+		t.Error("HTMX response should not contain DOCTYPE")
+	}
+	if !contains(body, "Page Not Found") {
+		t.Error("expected body to contain 'Page Not Found'")
+	}
+}
+
 func contains(s, substr string) bool {
 	return len(s) >= len(substr) && searchString(s, substr)
 }