docs: merge notification system docs into single Native-First guide

- Consolidate 5 notification-system-* files into doc/notification-system.md
- Add web-push cleanup guide and Start-on-Login glossary entry
- Configure markdownlint for consistent formatting
- Remove web-push references, focus on native OS scheduling

Reduces maintenance overhead while preserving all essential information
in a single, well-formatted reference document.

src/db/tables/settings.ts

@@ -29,6 +29,7 @@ export type Settings = {
   finishedOnboarding?: boolean; // the user has completed the onboarding process
 
   firstName?: string; // user's full name, may be null if unwanted for a particular account
+  hasBackedUpSeed?: boolean; // tracks whether the user has backed up their seed phrase
   hideRegisterPromptOnNewContact?: boolean;
   isRegistered?: boolean;
   // imageServer?: string; // if we want to allow modification then we should make image functionality optional -- or at least customizable