Add leads (liidit) tab for tracking potential customers

- New Liidit tab with table, search, add/edit/delete
- Lead fields: company, contact, phone, email, address, city, status, notes
- Status workflow: Uusi → Kontaktoitu → Kiinnostunut → Odottaa toimitusta
- Color-coded status badges
- Detail view with notes display
- "Muuta asiakkaaksi" converts lead to customer with pre-filled data
- Lead CRUD endpoints in api.php with changelog logging
- leads.json added to .gitignore

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

style.css

@@ -1053,6 +1053,41 @@ span.empty {
     background: #c0392b;
 }
 
+/* Lead status badges */
+.lead-status {
+    display: inline-block;
+    padding: 3px 10px;
+    border-radius: 12px;
+    font-size: 0.75rem;
+    font-weight: 600;
+    letter-spacing: 0.3px;
+}
+
+.lead-status-uusi {
+    background: #3498db;
+    color: #fff;
+}
+
+.lead-status-kontaktoitu {
+    background: #f39c12;
+    color: #fff;
+}
+
+.lead-status-kiinnostunut {
+    background: #2ecc71;
+    color: #fff;
+}
+
+.lead-status-odottaa {
+    background: #9b59b6;
+    color: #fff;
+}
+
+.lead-status-ei_kiinnosta {
+    background: #e8ebf0;
+    color: #888;
+}
+
 /* Changelog */
 .nowrap {
     white-space: nowrap;