feat: add a notification center and keep toasts longer

[andrebrait]

Toasts disappeared after 4 seconds, which is too short to read them or use their buttons. Once a toast was gone, it could not be brought back.

What changes

• Longer toasts. Success and info toasts stay 6 s, errors 10 s. Android's defaults (Toast 2 s / 3.5 s, Snackbar 1.5 s / 2.75 s) are too short for toasts that carry actions. Material recommends 4–10 s, and Windows and macOS banners stay about 5 s. An explicit timeout still wins, including 0 for toasts that stay until dismissed.
• Notifications tab in the right panel, next to Explorer and Git. It lists the last 100 toasts, newest first, each with its time. Entries keep their action buttons (for example the update toast's Copy), and can be dismissed one at a time or with Clear all. The pinned Explorer and Git tabs now share one PinnedTab component in TabBar.
• Unread badge on the panel toggle. The toggle keeps its icon. While there are unread notifications it shows their count, and tapping it opens the panel on the Notifications tab; otherwise it opens and closes the panel as before. Unread entries are highlighted while the tab is open and become read when you leave the tab or choose Mark all as read. Re-announcing a notice under the same id keeps its read state, so the update toast that re-fires on every tab focus doesn't re-badge.
• OS notifications are listed too. Task-completion notifications and the agent notify host tool are recorded with toastHistory.record(). A notify message is recorded even when the browser blocks notifications. A completion now falls back to an in-app toast when the permission prompt is dismissed or the Notification API is missing.
• No new top-bar button. An earlier revision of this PR added a bell to the top bar; it squeezed the session title on phones (195 px → 118 px at 390 px wide), so it moved into the panel.
• History is kept in memory only. Persisting it would drop the JSX descriptions that carry the action buttons.

Verification

• tsc --noEmit and eslint are clean. components/*.test.mjs and components/ui/toast.test.mjs: 210 pass, 0 fail. hooks/useAgentSession.rpc.test.mjs passes with the new notify test.
• New tests cover the 6 s / 10 s defaults and overrides, the 100-entry cap, replacement by reused id, distinct ids for recorded entries, subscriber notification, unread state, Mark all as read (including no notification when nothing is unread), and read state kept on re-announce.
• Headless Chromium at 390 px and 1280 px: the badge shows the unread count, tapping the toggle opens the Notifications tab, the session title is back to full width (194 px at 390 px), closing the panel marks entries read and removes the badge, Mark all as read and Clear all work, and keyboard focus stays in the panel after dismissing the last entry.

[kahme247]

Review — approve (SHIP)

Verified by independent review in a scratch worktree. Typecheck clean, toast tests 7/7, rpc hook tests 142/142, lint 0 errors.

Notification center (#230). Bounded both layers (4 on-screen, 100 history); no manual timers exist (dismissal owned by the base-ui Toast manager), so nothing to leak. Mark-read correct across tabs; roles correct. Two sub-minor notes for later: no aria-live on the list, and the panel toggle jumps to Notifications while unread. Mutation kills unexecuted (harness interruptions) — the assertions are direct value tests, high confidence, and the merge queue re-runs everything.

Label (#225). Four changed lines: drops the literal "+" from modelsConfig.addProvider in en/ja/zh while addModel keeps its "+"; both provider buttons render a separate <Plus> lucide icon, so the duplicate plus is genuinely gone in all locales. Nothing else to check.