feat(custom-model): detect llama-swap model conflicts before switching

Root-caused the user's earlier confusion ('the terminal says opus even though
something is waiting for llama to load'): llama.cpp runs exactly one model at
a time, and llama-swap unloads/reloads it on demand - a swap can take
anywhere from a few seconds to well over a minute, during which a session
looks indistinguishable from one still on the native backend.

1. Feature-detects llama-swap (vs. plain llama.cpp/any OpenAI-compatible
   server) via its own GET /running, which plain llama.cpp has no concept of
   at all. New GET /api/model-endpoints/:id/running-status route exposes this
   read-only, for the frontend's polling loop below.

2. Before applying a selection, POST /api/sessions/:id/custom-model now checks
   what llama-swap currently has loaded. If it differs from the requested
   model AND another live session's own customModel selection is actively
   using that loaded model, the apply is refused with a
   {requiresConfirmation, currentlyLoadedModel, affectedSessions} payload
   instead of silently switching. A "confirmed: true" field on the retry
   skips the check. Switching with nothing else affected proceeds
   immediately, no confirmation asked, only ever when there is something to
   warn about.

3. The frontend (runCustomModelEntry) shows a native confirm() naming the
   affected session(s) and the model they'd lose, matching this codebase's
   existing convention for this class of decision (delete case, kill
   session, etc.) rather than a new modal. On a successful apply the response
   also carries modelSwapInProgress; when true, a new _watchLlamaSwapLoading
   poll shows a sticky "Loading <model>..." toast via the new running-status
   route until llama-swap reports the target model ready (bounded at 2
   minutes), so a prompt sent mid-swap reads as "loading", never as silence
   or an answer from whatever was loaded a moment before.

Checks are read-only against llama-swap's own /running - never /props, which
takes a ?model= and can itself trigger a load as a side effect of asking.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RqZeHrRS6DYcGcGX2p9EwG

src/web/routes/session-routes.ts

@@ -55,6 +55,7 @@ import {
 } from '../schemas.js';
 import { readCustomModelHosts } from '../../custom-model-hosts.js';
 import { applyCustomModelInjection, removeConfigDir } from '../../custom-model-injection-apply.js';
+import { getLlamaSwapStatus } from './custom-model-routes.js';
 import { matchesPattern } from '../../config/cli-registry/patterns.js';
 import { ownerLayoutKey } from '../../tab-layout-persistence.js';
 import { TabLayoutValidationError } from '../../tab-layout.js';
@@ -1209,6 +1210,32 @@ export function registerSessionRoutes(
       return createErrorResponse(ApiErrorCode.NOT_FOUND, 'Model endpoint not found');
     }
 
+    // llama.cpp runs exactly one model at a time; llama-swap unloads and reloads it on
+    // demand, which can take anywhere from a few seconds to over a minute — long enough
+    // that a session mid-swap looks indistinguishable from one that never left the native
+    // backend. Feature-detected via llama-swap's own `GET /running` (a plain llama.cpp
+    // server has no such endpoint and reads as `isLlamaSwap: false` — nothing to check).
+    const swapStatus = await getLlamaSwapStatus(endpoint);
+    const currentlyLoaded = swapStatus.running.find((r) => r.state === 'ready')?.model ?? swapStatus.running[0]?.model;
+    const swapNeeded = swapStatus.isLlamaSwap && !!currentlyLoaded && currentlyLoaded !== body.modelId;
+
+    // Only ask when switching would actually take the model away from another session
+    // that is currently using it — never just because a swap is needed at all. `confirmed`
+    // (set by the caller after showing that warning once) skips asking again.
+    if (swapNeeded && !body.confirmed) {
+      const affectedSessions = [...ctx.sessions.values()]
+        .filter(
+          (s) =>
+            s.id !== session.id &&
+            s.customModel?.endpointId === endpoint.id &&
+            s.customModel?.modelId === currentlyLoaded
+        )
+        .map((s) => ({ id: s.id, name: s.name }));
+      if (affectedSessions.length > 0) {
+        return { requiresConfirmation: true, currentlyLoadedModel: currentlyLoaded, affectedSessions };
+      }
+    }
+
     // A CLI whose config alone cannot select the model also gets its `model` launch param
     // forced (pi/omp `custom/<id>`, grok's block name). The argv engine DROPS a token that
     // fails its pattern rather than quoting it, which would silently launch the CLI on its
@@ -1250,7 +1277,7 @@ export function registerSessionRoutes(
 
     const restarted = await session.restartCli();
     persistAndBroadcastSession(ctx, session);
-    return { customModel: session.customModel, restarted };
+    return { customModel: session.customModel, restarted, modelSwapInProgress: swapNeeded };
   });
 
   // ========== Delete Session ==========