v0.81.0: retire drive-activation banner; add standalone "Kiszolgáló újraindítása" button

In the intermediary-mount model an enrolled drive binds live into the running
guest (no reboot), so the "… meghajtó aktiválásra vár / Újraindítás most" banner
was an obsolete relic — also dead since v0.78 (pendingActivationDrives keyed on
the raw MountPath vs the now-stable sp.Path). Removed the banner block +
activatePendingDrives JS (settings.html), the PendingDrives feed (handlers.go),
and the dead pendingActivationDrives helper + its unused internal/system import.

Renamed handleStorageActivate -> HandleServerReboot (split out a testable
serverReboot core), removed the /api/storage/activate case, and mounted the
handler at the new non-storage route /api/server/reboot (RequireAuth+CsrfProtect).
The agent GuestReboot primitive is reused unchanged.

Added the standalone "Kiszolgáló újraindítása" settings card (sibling to the
controller-only "Vezérlő újraindítása"), reusing the pollRestart() loop.

Test: TestHandleServerReboot_CallsGuestReboot (fake diskAgent asserts GuestReboot
invoked once + 202). diskAgent/mockAgent gained GuestReboot.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PsnU2ASocYrvzqE82YDYW

controller/internal/web/handlers.go

@@ -903,9 +903,6 @@ func (s *Server) settingsData() map[string]interface{} {
 		storageViews = append(storageViews, view)
 	}
 	data["StoragePaths"] = storageViews
-	// Drives enrolled but not yet activated in the guest (slice 10 P2): they need the user-triggered
-	// "Újraindítás most" to take effect (the host-side live inject is blocked on an unprivileged guest).
-	data["PendingDrives"] = s.pendingActivationDrives()
 
 	// Recovery info for emergency section
 	data["RetrievalPassword"] = s.settings.GetRetrievalPassword()