[AI] Apply VRT threshold to electron config and drop redundant local override

- Remove `maxDiffPixels: 5` from `toMatchThemeScreenshots` — it duplicates
  the global config and obscured the fact that per-call options would
  override the global threshold if added there too.
- Mirror the threshold in desktop-electron's playwright config so its
  VRT screenshots are subject to the same sensitivity.
- Tighten the threshold comment: drop the PR/task reference per
  AGENTS.md and the worked-example numbers; keep the formula and the
  low-alpha rationale.

packages/desktop-client/e2e/fixtures.ts

@@ -63,7 +63,6 @@ export const expect = baseExpect.extend({
 
     const config = {
       mask: [target.locator('[data-vrt-mask="true"]')],
-      maxDiffPixels: 5,
     };
 
     const page: Page = 'page' in target ? target.page() : target;

packages/desktop-client/playwright.config.ts

@@ -26,13 +26,12 @@ export default defineConfig({
     // until layout provides width/height, and that can take >5s. Bumping
     // to 10s lets those assertions settle without per-test overrides.
     timeout: 10_000,
-    // `threshold` is pixelmatch's per-pixel YIQ-delta cutoff: a pixel only
-    // counts toward `maxDiffPixels` if its delta exceeds 35215 * threshold².
-    // Playwright's default (0.2 → cutoff 1408) silently swallows faint
-    // overlays — e.g. striping the transactions table with rgba(…, .15)
-    // produces deltas of ~270–320, so VRT reported 0 diff pixels and passed
-    // (PR #7841). Drop to 0.05 (cutoff ~88) so low-alpha tints are flagged
-    // while still leaving headroom for anti-aliasing noise.
+    // `threshold` is pixelmatch's per-pixel YIQ-delta cutoff — a pixel
+    // counts toward `maxDiffPixels` only if its delta exceeds
+    // 35215 * threshold². Playwright's 0.2 default lets faint color
+    // overlays (e.g. rgba(…, .15) row striping) slip through with 0
+    // reported diff pixels; 0.05 catches them while staying above
+    // anti-aliasing noise.
     toHaveScreenshot: { maxDiffPixels: 5, threshold: 0.05 },
   },
   webServer: process.env.E2E_START_URL

packages/desktop-electron/playwright.config.ts

@@ -16,7 +16,8 @@ export default defineConfig({
     ignoreHTTPSErrors: true,
   },
   expect: {
-    toHaveScreenshot: { maxDiffPixels: 5 },
+    // See desktop-client playwright.config.ts for the threshold rationale.
+    toHaveScreenshot: { maxDiffPixels: 5, threshold: 0.05 },
     timeout: 60000, // 60 seconds
   },
 });