feat: relocate vault backups to a managed folder with retention cap

[GFrancV]

Summary

• Adds src/main/vault/backups.ts: vault backups now live under userData/vault-backups/<sha256-of-vault-path>/ instead of being dropped next to the vault file forever, with a hardcoded retention cap (MAX_BACKUPS_PER_VAULT = 3 — oldest deleted first by mtime).
• confirmMigrationAndBackup (the shared helper from feat: add SQL schema version guard with migration confirmation parity #4) now calls createVaultBackup instead of an inline copyFileSync, so both header-version and schema-version migration backups land in the new location with descriptive names (<reason>-v<from>-to-v<to>-<timestamp>.nvx).
• Adds a "Open backups folder" button in Security settings, replacing the "Vault format" section — that section's manual "Upgrade vault format" action (a toast telling you to close/reopen the vault) is now dead code superseded by the automatic migration dialog from feat: add SQL schema version guard with migration confirmation parity #4/feat: relocate vault backups to a managed folder with retention cap #5, so it — and its only consumer, the useVaultCapabilities hook — are removed.
• Bug fix found during manual testing: vault:migration-confirmed/vault:migration-cancelled were synchronously nulling migrationBackupTimestamp right after resolving the pending migration promise. Since resolving a promise only schedules its continuation as a microtask (it doesn't run inline), this reset was winning the race every time — meaning the "create a backup" checkbox has silently never actually created a backup, since the confirmation dialog was wired up in feat: add SQL schema version guard with migration confirmation parity #4. Fixed by leaving the reset to the code that actually reads the flag.

Test plan

• Manual test via pnpm dev: triggered a schema migration with the backup checkbox checked — confirmed (after the race-condition fix) the backup file lands under %APPDATA%\Notvex Dev\vault-backups\<hash>\ with the new naming.
• "Open backups folder" with no backups yet → toast.error("No backups yet."), no explorer opens.
• "Open backups folder" after a backup exists → OS file explorer opens on the correct folder.
• Retention-trimming algorithm (keep 3 newest by mtime, delete the rest) verified in isolation with a throwaway script against 5 dummy files — logic matches createVaultBackup exactly.
• pnpm validate passes.

[github-actions]

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