feat(notifications): notify background permission requests

[wsp1911]

Summary

• Add a default-enabled setting for background permission request notifications.
• Notify for actionable core and ACP approval requests only while BitFun is not focused.
• De-duplicate request events and batch requests from the same session round within 400ms.
• Mount the listener at the app layout level so toolbar mode is covered, and update the cached setting immediately after toggling.

Fixes #1812

Type and Areas

Type: Feature

Areas: Rust core, desktop/Tauri, web UI, ACP, i18n

Motivation / Impact

Users can now receive a generic OS notification when an approval request needs attention while BitFun is in the background. The notification intentionally excludes tool arguments and other sensitive request details.

Verification

• pnpm --dir src/web-ui run test:run src/app/hooks/permissionRequestNotifyPolicy.test.ts src/flow_chat/services/flow-chat-manager/AcpPermissionToolCardModule.test.ts (6 tests passed)
• pnpm run type-check:web
• pnpm run i18n:audit
• cargo check -p bitfun-desktop
• cargo test -p bitfun-core permission_request_notifications_default_to_enabled
• pnpm run fmt:rs
• git diff --cached --check
• Manual: verified a background single-session permission request produces a notification.

Reviewer Notes

• Core and ACP requests share the same notification policy.
• Requests are grouped by sessionId + roundId; requests from separate sessions remain separate OS notification calls.
• Windows Toast acceptance should be checked using an installed NSIS build, not the unpackaged desktop:build:fast executable.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.