fix(display): keep blackout across reloads and follow the display's current assignment

A kiosk that reloaded during a blackout restored its last frame from local
state without the blackout, so the hidden slide flashed on the wall until the
server answered.

A display resolved its channel once, at registration. Moving it to another
group or audience, or deleting the channel it was on, never reached a running
screen: it kept showing the old channel, or fell into offline continuation
against a 404 forever. The display now fetches its state by viewer, so the
server resolves the current assignment on every fetch, and the heartbeat reply
names the assigned channel so a reassignment lands within one heartbeat. A
display removed from the roster registers again once, on backoff; only
Signage's own not-found means removal, so a foreign 404 can never cause a
registration loop. The channel route stays for the preview and for display
code a kiosk cached earlier. A replaced realtime subscription can no longer
schedule a stray resubscription when it reports closing.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

tests/browser/offline.test.ts

@@ -391,7 +391,7 @@ describe("a display that loses its network", () => {
         },
         {
           method: "GET",
-          path: "channels/chan-1/display-state",
+          path: /^(viewers\/[^/]+|channels\/chan-1)\/display-state$/u,
           respond: () => {
             if (!reachable) {
               throw new Error("network is gone");