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>

.gitignore

@@ -2,6 +2,7 @@ data/customers.json
 data/users.json
 data/changelog.json
 data/archive.json
+data/leads.json
 data/reset_tokens.json
 data/login_attempts.json
 data/backups/