feat(signalr): show the server's restart reasons to admins

[revam]

Why

The server now keeps why it needs a restart (GET /api/v3/Init/RestartReasons, and the admin-only restart feed). This is the first part that needs a newer server, so it raises the minimum to 6.0.0-dev.495.

What changed

• RestartNotice (replacing the toast signalr.ts raised, now mounted in MainPage): lists each reason's plugin or setting and why, beside the configuration:requiresRestart notice, follows restart:reasonsChanged and closes once no reason is left.
• MainPage waits for the current user before connecting (useFreshCurrentUser, shared with RestartNotice), and MAINPAGE_LOADED carries isAdmin; only admins join the restart feed and call the route.
• The configuration:requiresRestart flag lives in the mainpage slice, so it resets on logout.
• vite.config.mjs: minimum server version 6.0.0-dev.479 → 6.0.0-dev.495.
• AGENTS.md: notes the admin-only feed.

Note for review: Permissions: the feed and route are admin-only on the server; the WebUI only skips them for other users. The SignalR connection now starts after the current user is loaded. The minimum server version rises.

Where to look first

• src/components/RestartNotice.tsx: the new prompt.
• src/core/signalr/signalr.ts: joining the feed for admins only.

Feedback wanted

Whether waiting for the current user before connecting SignalR is acceptable.

Stack

Part 6 of 12 in the split of #1477. Below: Part 5, two-parameter filter conditions. Above: Part 7, the scheduled actions page.

Testing

pnpm tscheck, pnpm lint, pnpm test and pnpm build pass at this branch's tip. Manual, on 6.0.0-dev.495: as an admin, change a setting that needs a restart and see the reason listed; as a non-admin, no restart feed or RestartReasons call.

[hidden4003]

Should be available to all users not just admins