Establish a distinct workbench visual frame

Why: Spacing-only refinements still looked like the original flat gray workspace and did not satisfy the approved visual redesign.
Evidence: 272 unit tests, 16 browser scenarios across 320-1920px, and the production build pass locally.
Impact: Gives navigation, authored messages, assistant results, and work details clearly different visual surfaces while preserving behavior.
Refs: Agent-System-Dev feature/workbench-visual-redesign

e2e/work-companion.playwright.ts

@@ -159,6 +159,34 @@ test('uses a focused reading column and a compact authored-message shape', async
   expect(authoredBox!.width).toBeLessThanOrEqual(680)
   expect(authoredBox!.x + authoredBox!.width).toBeCloseTo(answerBox!.x + answerBox!.width, 0)
 
+  const surfaces = await page.evaluate(() => {
+    const pick = (selector: string) => {
+      const element = document.querySelector(selector)
+      if (!element) throw new Error(`missing ${selector}`)
+      const style = getComputedStyle(element)
+      return {
+        background: style.backgroundColor,
+        color: style.color,
+        borderRadius: Number.parseFloat(style.borderRadius),
+        paddingLeft: Number.parseFloat(style.paddingLeft),
+      }
+    }
+    return {
+      chrome: pick('.companion-topbar'),
+      context: pick('.left-nav'),
+      conversation: pick('.main'),
+      answer: pick('.msg.assistant'),
+      authored: pick('.msg.user .bubble'),
+    }
+  })
+  const rgb = (value: string) => value.match(/[\d.]+/g)?.slice(0, 3).map(Number) ?? [0, 0, 0]
+  const distance = (a: string, b: string) => rgb(a).reduce((sum, value, index) => sum + Math.abs(value - rgb(b)[index]), 0)
+  expect(distance(surfaces.chrome.background, surfaces.conversation.background)).toBeGreaterThan(300)
+  expect(distance(surfaces.context.background, surfaces.conversation.background)).toBeGreaterThan(300)
+  expect(distance(surfaces.authored.background, surfaces.answer.background)).toBeGreaterThan(300)
+  expect(surfaces.answer.borderRadius).toBeGreaterThanOrEqual(12)
+  expect(surfaces.answer.paddingLeft).toBeGreaterThanOrEqual(20)
+
   const activeCard = page.locator('.right-panel .acard.status-active')
   await expect(activeCard).toBeVisible()
   const cardStyle = await activeCard.evaluate(element => {