Add a Notification Center panel to review missed completion notifications

### Before filing
- [x] I searched [open and closed issues](https://github.com/block/berd/issues?q=is%3Aissue) for duplicates.
- [x] I am using a recent [release](https://github.com/block/berd/releases) and this doesn't already exist.
- [x] This is one request, not several bundled together.

### Closest existing issue
none found (related but distinct: the toast-duration request I'm filing separately, since a longer-lived toast doesn't solve losing notifications you missed entirely)

### Is this new, or an improvement?
New capability — Berd can't do this at all today

### The problem, in your terms
Completion toasts are ephemeral — once a toast disappears (missed, dismissed, or timed out), there's no way to go back and see what notifications fired earlier in the session, or across sessions. If I step away from my desk for ten minutes while several agent turns complete, I come back to no record of what happened or which sessions need my attention, short of manually reopening every session tab to check.

### What you do today
I manually click through open sessions/tabs one at a time to check which ones finished, errored, or are waiting on input. There's no single place to see "what did I miss."

### What you'd like to see
A "Notification Center" — a small icon in the top bar (next to the existing search icon) that opens a panel/popover listing recent notifications (session name, what happened, timestamp), so you can review anything you missed and jump straight to the relevant session.

### Why this belongs in Berd itself
This needs access to the app shell's global notification/toast dispatch across all sessions and windows, plus top-bar UI real estate — that's core app-shell surface, not something an extension or skill can hook into from the agent/session layer.

### Non-goals
- Not asking for persistent notification history across app restarts (e.g. stored to disk) — an in-memory list scoped to the current app session is enough for the initial ask.
- Not asking to replace or remove the existing toast pop-ups — this is additive, a way to review what toasts already showed.
- Not asking for notification filtering/search/preferences in v1 — just a chronological list.

### Alternatives you considered
- Increasing toast duration (filed as a separate request) — helps catch things in the moment, but doesn't help if you're away when it fires, so it doesn't replace this.
- Manually re-checking each session tab — the current workaround, doesn't scale past 2-3 concurrent sessions.

Start by locating the app shell's global notification/toast dispatch and the top-bar area next to the existing search icon, then trace how completion notifications span sessions and windows. Done means an in-memory Notification Center lists recent notifications chronologically with session name, event, and timestamp, and lets users jump to the relevant session without removing existing toasts.

Written by the indexing model from the issue text.