feat(canvas): add shared workspace access and controls

frontend/src/ui/styles.css

@@ -1295,6 +1295,21 @@ button:disabled {
   grid-template-columns: minmax(0, 1fr);
 }
 
+.workspace-shell.is-canvas-only .workspace-topbar {
+  display: none;
+}
+
+.workspace-shell.is-canvas-only .workspace-main {
+  height: 100vh;
+  margin-top: 0;
+}
+
+@supports (height: 100dvh) {
+  .workspace-shell.is-canvas-only .workspace-main {
+    height: 100dvh;
+  }
+}
+
 .workspace-stage-title {
   position: absolute;
   top: 0;
@@ -1334,6 +1349,20 @@ button:disabled {
   background: transparent;
 }
 
+.canvas-share-control {
+  position: absolute;
+  z-index: 46;
+  top: 11px;
+  right: 58px;
+}
+
+.canvas-share-control .assistant-header-icon {
+  border: 1px solid rgba(27, 31, 37, 0.08);
+  background: rgba(255, 255, 255, 0.88);
+  box-shadow: 0 2px 8px rgba(17, 24, 39, 0.07);
+  backdrop-filter: blur(12px);
+}
+
 .canvas-collapsed-credit {
   height: 24px;
   display: inline-flex;
@@ -1611,6 +1640,14 @@ button:disabled {
   transform-origin: 0 0;
 }
 
+.workspace-readonly-shield {
+  position: absolute;
+  inset: 0;
+  z-index: 39;
+  cursor: default;
+  background: transparent;
+}
+
 .canvas-node {
   position: absolute;
   overflow: hidden;
@@ -3027,6 +3064,24 @@ button:disabled {
   background: #303030;
 }
 
+.canvas-statusbar.is-disabled,
+.workspace-toolbar.is-disabled {
+  opacity: 0.46;
+  filter: grayscale(0.45);
+  pointer-events: none;
+}
+
+.workspace-toolbar.is-disabled button,
+.workspace-toolbar.is-disabled button:hover,
+.workspace-toolbar.is-disabled button.active,
+.canvas-statusbar.is-disabled button,
+.canvas-statusbar.is-disabled button:hover,
+.canvas-statusbar.is-disabled button.active {
+  color: #9299a3;
+  background: transparent;
+  cursor: default;
+}
+
 .canvas-statusbar span {
   min-width: 34px;
   text-align: center;
@@ -3527,6 +3582,13 @@ button:disabled {
   background: rgba(82, 109, 135, 0.15);
 }
 
+.assistant-footer-dock.is-disabled {
+  opacity: 0.48;
+  filter: grayscale(0.55);
+  pointer-events: none;
+  user-select: none;
+}
+
 .assistant-history-trigger {
   width: 18px;
   min-width: 18px;