fix(frontend): render project watermark icons full-size above the composer with detail-preserving grayscale — #353 (QA verte)

Cause: the monochrome-first treatment from #351 flattened the emoji
artwork through an alpha mask (transparent fill + zero-offset
text-shadow), dropping internal details (the ghost lost its tongue and
eyes), and the watermark box did not occupy the full space available
above the chat composer.
Fix: new .project-cell-illustration-area container (container-type:
size) wrapping the scroll zone so the absolute watermark spans the
whole region between the agent toolbar and the composer; glyph keeps
the OS full-colour emoji artwork as source and is neutralised with a
non-destructive CSS filter (grayscale(1) contrast(0.92)) so internal
tones and details stay visible in black and white; glyph sized to
min(100cqw - 8px, 100cqh - 12px); text-variation selector U+FE0E
stripping removed (full emoji preserved) and project-theme tint
plumbing (accessibleProjectAccent/onProjectThemeChanged) dropped as
superseded by grayscale; empty label and overlay z-layering untouched.
QA verte reconfirmée par Git: npm test -- CustomAgentChatView.test.tsx
projectIcons.test.tsx 96/96, npm run typecheck OK, npm run build OK,
npm test 1396/1396, git diff --check OK.

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

frontend/src/shared/styles/theme.css

@@ -107,23 +107,21 @@
     container-type: size;
   }
 
+  .project-cell-illustration-area {
+    container-type: size;
+  }
+
   .project-cell-illustration {
     inset: 6px 4px;
-    color: transparent;
-    opacity: 0.045;
+    opacity: 0.085;
   }
 
-  .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. */
+  /* Keep the operating system's full-colour emoji artwork as the source image:
+     grayscale preserves internal tones and details that a monochrome alpha
+     mask (for example color: transparent + text-shadow) would flatten. */
   .project-cell-illustration__glyph {
-    color: transparent;
-    -webkit-text-fill-color: transparent;
-    text-shadow: 0 0 0 var(--project-illustration-tint, var(--color-muted));
+    filter: grayscale(1) contrast(0.92);
+    font-size: min(calc(100cqw - 8px), calc(100cqh - 12px)) !important;
   }
 
   .project-cell-empty-label {