fix(frontend): expand empty-state watermark to full cell and tint monochrome project icons by project color — #351 (QA verte)

Cause: the empty-state watermark filled only 92% of a centered box and
kept residual palette colors from host emoji fonts, non-compliant with
the monochrome-first project icon rule (#351 user capture).
Fix: illustration spans the cell with 6px/4px insets and an unclipped
glyph at min(88cqw, 88cqh); emoji painted as monochrome silhouettes via
transparent fill + zero-offset text-shadow tint
(--project-illustration-tint) derived from accessibleProjectAccent when
a project color exists, neutral muted fallback otherwise; empty label
anchored bottom-center and hidden first on constrained containers
(@container 280px width / 240px height); overlay priorities, composer
and z-layering untouched.
QA verte reconfirmée par Git: npm test -- CustomAgentChatView.test.tsx
80/80, git diff --check; QA: ProjectSettingsPanel 12/12, typecheck,
build OK (warning Vite chunk >500 kB préexistant).

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

frontend/src/shared/styles/theme.css

@@ -103,6 +103,51 @@
 }
 
 @layer components {
+  .custom-agent-chat-view {
+    container-type: size;
+  }
+
+  .project-cell-illustration {
+    inset: 6px 4px;
+    color: transparent;
+    opacity: 0.045;
+  }
+
+  .project-cell-illustration[data-color-mode="project"] {
+    opacity: 0.065;
+  }
+
+  /* Emoji fonts may ignore the text-variation selector and retain their own
+     palette. Painting a zero-offset text shadow through the glyph alpha mask
+     guarantees one semantic project tint without depending on the host font. */
+  .project-cell-illustration__glyph {
+    color: transparent;
+    -webkit-text-fill-color: transparent;
+    text-shadow: 0 0 0 var(--project-illustration-tint, var(--color-muted));
+  }
+
+  .project-cell-empty-label {
+    position: relative;
+    z-index: 1;
+    max-width: min(34rem, 88%);
+    margin: auto auto 8px;
+    text-align: center;
+  }
+
+  /* Preserve the illustration when a split cell becomes constrained; the
+     explanatory copy is the first element to yield. */
+  @container (max-width: 280px) {
+    .project-cell-empty-label {
+      display: none;
+    }
+  }
+
+  @container (max-height: 240px) {
+    .project-cell-empty-label {
+      display: none;
+    }
+  }
+
   .stopped-agent-cell {
     position: absolute;
     inset: 0;