Settings: change tin notes/notes, label fix, placeholder GL columns

Change tin targets:
- Add £5/£10/£20/£50 note rows (qty in individual notes, not bags)
- Fix label: (= £40.00 @ £20.00 each) instead of (= £40.00 each)
- Add per-denomination Notes text field (stored as change_tin_notes setting)
- Table layout replaces 2-col grid for all 10 denominations

Float count form:
- Notes denominations show a single Qty input; coins keep Bags + Loose
- Change order correctly shows "notes" vs "bags" needed per denomination

Sales breakdown columns:
- Add placeholder column button (gl_code='', labelled PLACEHOLDER)
- Add delete (×) button on every column row
- Fix key={i} to support multiple placeholders with same empty gl_code

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

backend/src/db.js

@@ -143,7 +143,8 @@ export async function initDb() {
       ('petty_cash_float',         '200.00'),
       ('till_float_target',        '0.00'),
       ('sales_breakdown_columns',  '[]'),
-      ('change_tin_breakdown',     '{"50.00":0,"20.00":0,"10.00":0,"5.00":0,"2.00":20,"1.00":20,"0.50":10,"0.20":10,"0.10":5,"0.05":5}')
+      ('change_tin_breakdown',     '{"50.00":0,"20.00":0,"10.00":0,"5.00":0,"2.00":20,"1.00":20,"0.50":10,"0.20":10,"0.10":5,"0.05":5}'),
+      ('change_tin_notes',         '{}')
     ON CONFLICT (key) DO NOTHING
   `)
 }

backend/src/routes/settings.js

@@ -4,7 +4,7 @@ import { testConnection, fetchGlAccountsGrouped } from '../lib/newbook.js'
 
 const ALL_KEYS = [
   'default_report_days', 'petty_cash_float', 'till_float_target',
-  'sales_breakdown_columns', 'change_tin_breakdown',
+  'sales_breakdown_columns', 'change_tin_breakdown', 'change_tin_notes',
 ]
 
 export async function settingsRoutes(app) {