Add in-product notification center

[CORCTON]

Summary

• add a navbar notification center with a red-dot unread indicator, cursor pagination, notification details, and internal actions
• return unread notifications before read notifications in one flat list without separate sections or pagination
• derive hasUnread from the notification list response and refresh on sign-in and notification-center open without polling
• keep notifications in place after marking them read and temporarily disable page loading while that update is pending
• guard user switches, stale requests, and cursor duplicates after notifications move into the read group on subsequent fetches
• add notification API coverage, accessibility metadata, and OpenAPI documentation
• keep Copilot below the standard modal layer so notification dialogs remain modal

Backend: goplus/builder-backend#355

This PR provides reusable notification UI. Feedback replies for #1789 will become the first producer in a separate PR.

Testing

• notification API and component tests: 13 passed
• npm run type-check
• npm run lint -- --no-fix

[CORCTON]

/review -claude

[fennoai]

Review: in-product notifications

Solid, well-structured feature. The stale-response guarding (requestSequence / unreadRequestSequence) and the isCurrentNotification() membership+timestamp check make the optimistic mark-as-read and user-switch races robust, and the tests cover them directly. Vue reactivity of the mutated array items is correct, teardown of the timer and visibilitychange listener is clean, and the OpenAPI contract matches the frontend types (field names, readAt nullability, optional actionPath, 1-based pageIndex).

Findings below are mostly low-severity; the unreadCount overwrite on "Load more" (P2) is the one worth confirming as intentional. No blocking issues.

Also verified as fine (no change needed): the const: true on the PATCH read body is valid OpenAPI 3.1; the unconditional interval timer is harmless for signed-out users since refreshUnreadCount early-returns; the poll + visibility overlap is deduped by the sequence guard; and notification.body/title are rendered with escaped {{ }} interpolation (no v-html).

[fennoai]

Review: in-product notifications (follow-up)

One additional low-severity note that couldn't be attached to the previous review.