refactor(kbhttp): one reconciling goroutine for both local servers

[chrisnojima]

Stack 4/9, splitting #29637. Parent: #29651.

Why

The local http server serves chat attachments, avatars and maps to the app's own webviews and image components. When it is down, images silently fail. On master there are two separate implementations of "keep a local server alive across app-state changes" — the service's and KBFS's — each with its own monitor, lock and restart path, and neither recovers from the server dying on its own.

Master bugs this fixes

• A dead server stays dead. startHTTPSrv starts the server and returns; nothing watches for Serve exiting. If it exits unexpectedly, the server stays down until the next app-state transition happens to restart it. Until then every attachment, avatar and map fails to load.
• The server is torn down on INACTIVE, so a transient state like Control Center or a system alert kills image loading. With feat(lifecycle): derive the app state from a lifecycle controller #29651 the app reports INACTIVE far more often (including every cold launch and resume), which would make this much worse.
• Two duplicate implementations. KBFS's libhttpserver has its own restart(), monitorAppState(), lock and cancel func, so a fix has to be made twice and the two have already drifted — KBFS's monitor only ever restarted, never stopped.
• config.go polls Addr() for up to 2 seconds to learn the address instead of asking once.
• The server does not keep its last bound address while stopped, so callers that need a URL while backgrounded get nothing.

What this changes

One reconciling goroutine owns both servers. manager.New takes appState/nextAppState as plain function parameters rather than an interface — that is what lets KBFS pass its own AppStateUpdater and the service pass MobileAppState without a shared type. reconcile tears the server down only in BACKGROUND, and only where stopInBackground. The server restarts after an unexpected Serve exit, keeps its last bound address while stopped, and rebinds when leaving the background. libhttpserver deletes its own machinery and delegates. config.go makes one Info() call.

httpSrv moves from NewService to SetupCriticalSubServices because the new Srv reads the notify router once at construction, and the router does not exist yet in NewService.

Also includes the localhost-src slice: a retried image src is repointed at the current server address and token, so an image baked with an old address recovers instead of failing forever.

One fix beyond a straight port of #29637

#29637 repoints any http://127.0.0.1:PORT src. KBFS serves /files/ from a second local server on its own port with its own independently-rotated token, so that rewrote KBFS URLs to the chat server's address and token — turning a transient image failure into a permanent wrong-port/403. Before this PR the retry only appended a cache-buster, so the regression would have been introduced here. Repointing is now gated on the service's own endpoints (/at, /av, /map); everything else still gets the cache-buster. Reachable from the KBFS file preview, which passes these URLs to ZoomableImage. Regression test added.

Verification

go build, go vet, gofmt, golangci-lint --new-from-rev master 0 issues. kbhttp, libhttpserver and attachment tests pass, including under -race and repeated runs. yarn lint:all clean, unit suite green.

Worth a decision during review: stopInBackground means KBFS's server is now torn down in BACKGROUND and no longer binds at New() on mobile, where the initial state is BACKGROUND — so Server.Address() and GetGUIFileContext fail until the first foreground. That is a real behaviour change from master, where it was never stopped.