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/features/agents/CustomAgentChatView.tsx

@@ -15,6 +15,7 @@ import {
   useRef,
   useState,
   type ClipboardEvent,
+  type CSSProperties,
   type KeyboardEvent,
   type MouseEvent as ReactMouseEvent,
   type RefObject,
@@ -45,7 +46,11 @@ import {
 } from "@/features/projects/projectCliInteractionActivity";
 import { Button, EntryTimestamp, Spinner, cn, normalizeTimestamp } from "@/shared";
 import { onProjectIconChanged, ProjectIcon } from "@/features/projects/projectIcons";
-import { onProjectThemeChanged, validThemeColor } from "@/features/projects/projectTheme";
+import {
+  accessibleProjectAccent,
+  onProjectThemeChanged,
+  validThemeColor,
+} from "@/features/projects/projectTheme";
 import type {
   ChatAttachmentInput,
   ChatDraftScope,
@@ -2504,26 +2509,28 @@ export function CustomAgentChatView({
     <div
       ref={chatRootRef}
       data-testid="custom-agent-chat-view"
-      className="relative z-0 flex h-full min-h-0 flex-col overflow-hidden bg-surface text-content"
+      className="custom-agent-chat-view relative z-0 flex h-full min-h-0 flex-col overflow-hidden bg-surface text-content"
     >
       <div
         data-testid="project-icon-watermark"
         data-color-mode={projectThemeColor ? "project" : "monochrome"}
         aria-hidden="true"
-        className="pointer-events-none absolute inset-0 z-0 flex items-center justify-center overflow-hidden text-content opacity-[0.055]"
+        className="project-cell-illustration pointer-events-none absolute z-0 flex items-center justify-center overflow-hidden"
         style={{
           pointerEvents: "none",
-          containerType: "size",
-          color: projectThemeColor ?? undefined,
-        }}
+          "--project-illustration-tint": projectThemeColor
+            ? accessibleProjectAccent(projectThemeColor)
+            : "var(--color-muted, #9aa7b4)",
+        } as CSSProperties}
       >
         <ProjectIcon
           icon={projectIcon}
           fallbackLabel={projectFallbackLabel}
           surface="watermark"
-          size="92%"
+          size="100%"
           decorative
-          style={{ fontSize: "min(34cqw, 34cqh)", maxWidth: "92cqh", maxHeight: "92cqw" }}
+          className="project-cell-illustration__glyph"
+          style={{ fontSize: "min(88cqw, 88cqh)" }}
         />
       </div>
       <div
@@ -2727,7 +2734,7 @@ export function CustomAgentChatView({
             </div>
           </div>
         ) : displayTurns.length === 0 ? (
-          <p className="m-auto text-xs text-muted">
+          <p className="project-cell-empty-label text-xs text-muted">
             {t("custom.empty")}
           </p>
         ) : (