Frontend aggregate state: SPA-side derive + checker fixes

The web UI showed the wrong up/down state for frontends whose pool
composition had been touched by a mix of runtime disable/enable and
weight changes: a frontend with every backend at effective_weight=0
would still display "up", while a sibling frontend with a serving
fallback backend would display "down". Two independent bugs, each
fixed on its own layer.

On the fast path (healthCheckEqual returns true), Reload did
`w.entry = b`, blindly replacing the runtime worker entry with the
fresh YAML record. YAML's default for Enabled is true, so any
backend the operator had runtime-disabled would have its Enabled
flag silently reset while the worker's backend.State stayed at
StateDisabled. Subsequent EnableBackend calls then early-returned
on `if w.entry.Enabled` and never transitioned the state machine
— the CLI reported "enabled, state is 'disabled'" and the backend
was permanently stuck.

Fix: preserve w.entry.Enabled across the fast-path replacement.

    runtimeEnabled := w.entry.Enabled
    w.entry = b
    w.entry.Enabled = runtimeEnabled

Runtime operator state now outlives config reloads. On the worker-
restart path (different health check) the new worker is
structurally fresh and the YAML's Enabled is still authoritative.

Both methods used `w.entry.Enabled` as their idempotency check,
which meant a stuck `Enabled=true, State=disabled` combo couldn't
be repaired even after the Reload fix (existing bad state had to
survive the upgrade). Switched both methods to key on
`w.backend.State`:

 - DisableBackend: if state == StateDisabled, sync the flag but
   don't emit a redundant transition; otherwise do the full
   state transition + flag flip + worker cancel.
 - EnableBackend: if state != StateDisabled, sync the flag but
   don't emit a redundant transition; otherwise do the full
   transition + flag flip + probe-goroutine restart.

Either method will now unstick any inconsistency between the
flag and the state machine — future drift from a panic, a new
code path we haven't thought of, or existing already-stuck
backends from before this commit are all repaired on the next
enable/disable call.

Changing a backend's weight can flip a frontend between up and
down (e.g. zeroing the last non-zero-weighted backend in the
active pool), but SetFrontendPoolBackendWeight never called
updateFrontendState, so the checker's cached frontend state
would drift from reality until the next genuine backend
transition happened to trigger a recompute. The symptom was
"show frontends nginx-ip4-http" reporting up even with every
effective_weight=0.

Fix: call c.updateFrontendState(frontendName, fe) after the
weight mutation, under the same lock. The recompute emits a
FrontendEvent transition if the aggregate flipped, so any
WatchEvents consumer picks up the change live.

stores/state.ts recomputeEffectiveWeights is renamed and
extended to recomputeDerivedState, which now also writes
fe.state using the same rule as health.ComputeFrontendState:
unknown if no backends or all unknown, up if any effective
weight > 0, down otherwise. Called from every mutation path
(replaceAll, replaceSnapshot, applyBackendTransition,
applyConfiguredWeight) so the SPA is authoritative for *display*
state and doesn't inherit any staleness the server's cached
frontendStates map might have.

applyFrontendTransition is now a no-op for the state field —
the server's `to` value is no longer trusted because
recomputeDerivedState walks the local backends array on every
update and produces a fresh, correct answer. The reducer is kept
as a named function so sse.ts's dispatch table still has a
landing spot for "frontend" events (they still feed the
DebugPanel via pushEvent); the empty body is deliberate, not a
bug — a comment at the top spells it out.

cmd/frontend/web/dist/index.html

@@ -4,7 +4,7 @@
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     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>maglev</title>
-    <script type="module" crossorigin src="/view/assets/index-BBNMNdtq.js"></script>
+    <script type="module" crossorigin src="/view/assets/index-C-XMkBf5.js"></script>
     <link rel="stylesheet" crossorigin href="/view/assets/index-CxDuAfMR.css">
   </head>
   <body>