feat(model-server): afficher le téléchargement du modèle llamacpp au démarrage (#54)

Ajoute un handle du téléchargement des modèles lors du démarrage de
llamacpp : le domaine et l'application émettent la progression de
téléchargement du modèle, relayée en événement côté app-tauri, et l'UI
l'affiche via un badge de lancement et un overlay de cellule pendant que
le serveur de modèle démarre.

Backend (B1) : progression de téléchargement dans domain/application,
relais d'événement app-tauri, couverture de tests.
Frontend (F1) : modelServerLaunch, badge et overlay LayoutGrid, tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/features/layout/LayoutGrid.tsx

@@ -38,6 +38,10 @@ import {
   TargetAnnouncementsOverlay,
   useTargetAnnouncements,
 } from "@/features/announcements";
+import {
+  modelServerOverlayText,
+  useModelServerLaunchState,
+} from "@/features/agents";
 import { leaves, normalizeWeights, resizeAdjacent } from "./layout";
 import { useLayout, type LayoutViewModel } from "./useLayout";
 
@@ -367,6 +371,18 @@ function LeafView({
   // write-portal veil can be mutually excluded with the F3 overlay (exactly one
   // full-cell veil at a time, F3 first). Same source the overlay itself reads.
   const { active: busyActive } = useTargetAnnouncements(agentId ?? "");
+  // Ticket #54 — local model-server launch state for this cell's pinned agent.
+  // Correlated `agentId → profileId → opencode.localModelServerId → status` via
+  // the shared hook (same reduction as `useAgents`). While the bound server is
+  // still preparing (downloading a model / starting / probing), a full-cell
+  // overlay replaces the timeout/error the empty terminal used to show. It has
+  // the HIGHEST veil priority: nothing else in the cell is usable until the
+  // server is ready, so it suppresses the write-portal and F3 veils.
+  const { statusForAgent } = useModelServerLaunchState(projectId);
+  const pinnedAgent = agentId
+    ? agents.find((a) => a.id === agentId)
+    : undefined;
+  const modelServerOverlay = modelServerOverlayText(statusForAgent(pinnedAgent));
   const agentWork = agentId
     ? workState?.agents.find((row) => row.agentId === agentId)
     : undefined;
@@ -894,7 +910,8 @@ function LeafView({
             is being injected into the agent's PTY, a grey veil with a centred
             message sits above the terminal. Only ever shown for an agent cell —
             and never together with the F3 overlay (exactly one veil, F3 first). */}
-        {shouldShowWritePortalVeil(agentId != null, Boolean(overlay), busyActive) && (
+        {!modelServerOverlay &&
+          shouldShowWritePortalVeil(agentId != null, Boolean(overlay), busyActive) && (
           <div
             data-testid="write-portal-overlay"
             role="status"
@@ -931,9 +948,46 @@ function LeafView({
             busy state (agentBusyChanged + read-model hydration), never by the raw
             PTY — it retracts at idle even when a turn ends without a completion
             event. Self-guards on `active` (busy) and renders null otherwise. */}
-        {agentId != null && (
+        {!modelServerOverlay && agentId != null && (
           <TargetAnnouncementsOverlay projectId={projectId} agentId={agentId} />
         )}
+        {/* Ticket #54 — model-server launch veil. Top-priority full-cell overlay
+            (suppresses the write-portal + F3 veils above): while the pinned
+            agent's bound local server is downloading its model / starting /
+            probing, the empty terminal is covered rather than left to time out.
+            Retracts at `ready`/`failed` (the predicate returns null then). */}
+        {modelServerOverlay && (
+          <div
+            data-testid="model-server-overlay"
+            role="status"
+            aria-live="polite"
+            aria-label="model server launch"
+            style={{
+              position: "absolute",
+              inset: 0,
+              zIndex: CELL_Z.veil,
+              display: "flex",
+              alignItems: "center",
+              justifyContent: "center",
+              background: "rgba(0, 0, 0, 0.55)",
+              color: "var(--color-content, #e0e0e0)",
+              fontSize: 13,
+              pointerEvents: "none",
+              userSelect: "none",
+            }}
+          >
+            <span
+              style={{
+                background: "var(--color-surface, #1e1e1e)",
+                border: "1px solid var(--color-border, #3a3a3a)",
+                borderRadius: 4,
+                padding: "6px 12px",
+              }}
+            >
+              {modelServerOverlay}
+            </span>
+          </div>
+        )}
       </div>
       {busyNotice && (
         <div