Enhance newsletter generation and dashboard functionality

- Introduce a new period selection feature in the dashboard for generating newsletters, allowing users to choose between "last month" and "last X days."
- Refactor newsletter generation logic to support filtering by article type (new, edited, or all) and improve the handling of filters in the dashboard.
- Update the dashboard template to include new input fields for period and article type, enhancing user experience.
- Improve CSS styles for send notifications and sections in the dashboard, providing clearer feedback on newsletter sending status.

app/static/css/main.css

@@ -510,6 +510,42 @@ tbody tr:hover { background: #FAFAFA; }
     display: block;
 }
 
+.send-section {
+    margin-top: 1.25rem;
+    padding: 1rem 1.1rem;
+    border: 1px solid var(--tk-border);
+    border-left: 4px solid var(--tk-orange);
+    border-radius: var(--radius);
+    background: var(--tk-gray-light);
+}
+
+.send-banner {
+    margin-top: 1rem;
+    padding: 0.75rem 1rem;
+    border-radius: var(--radius);
+    font-size: 0.9rem;
+    font-weight: 600;
+    border: 1px solid transparent;
+}
+
+.send-banner-sent {
+    background: #E7F5EC;
+    color: #1E7A43;
+    border-color: #B7E0C6;
+}
+
+.send-banner-failed {
+    background: #FDEAEA;
+    color: #B42318;
+    border-color: #F5C2C0;
+}
+
+.send-banner-skipped {
+    background: var(--tk-orange-light);
+    color: var(--tk-orange-dark);
+    border-color: #F5C9A6;
+}
+
 .textarea-actions {
     display: flex;
     gap: 0.5rem;