Today view and Inbox section: dynamic query, not stored sections

- ListTodayView() on backend: queries nodes created or updated today
  using local timezone day boundaries
- todayBoundaries() helper returns start/end of current day in RFC3339
- Section validation: Create() rejects today and inbox as node sections
- validSections moved from repository.go to types.go with IsValidSection
  and IsServiceSection helpers
- Frontend selectSection('today') calls ListTodayView instead of
  ListNodesBySection('today')
- FAB (create node) hidden in today and inbox sections
- CreateNode in app.go guarded against today/inbox sections
- types.go: today/inbox defined as service sections (sidebar only)

frontend/src/wailsjs/go/main/App.js

@@ -6,6 +6,10 @@ export function ListSections() {
     return window['go']['main']['App']['ListSections']();
 }
 
+export function ListTodayView() {
+    return window['go']['main']['App']['ListTodayView']();
+}
+
 export function ListNodesBySection(arg1) {
     return window['go']['main']['App']['ListNodesBySection'](arg1);
 }