fix(task): keep priority menu above task dialogs

[WdBlink]

💡 Change Scope

• Small Change: Fast feedback, localized UI/logic, low-risk.
• Large Change: New feature, cross-boundary logic, runtime-risk (persistence, IPC, lifecycle, recovery).

📝 What Does This PR Do?

Keeps the Task Creator and Task Editor priority menus above their modal backdrops by opting those two CoveSelect instances into the existing dialog-popover layer.

This is intentionally local: the default CoveSelect popover layer and global z-index tokens remain unchanged. Regression coverage verifies both task-window call sites, confirms standalone selects retain the default layer, and exercises the Task Editor menu in Electron at a constrained viewport, including stacking, clipping, selection, and persistence.

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🏗️ Large Change Spec (Required if "Large Change" is checked)

Not applicable — this is a localized presentation-layer fix with no state, API, IPC, persistence, or lifecycle changes.

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✅ Delivery & Compliance Checklist

• My code passes the ultimate gatekeeper: pnpm pre-commit is completely green.
• I have signed the CLA if required (see CLA.md).
• I have included new tests to lock down the behavior (or explicitly stated why it's untestable).
• I have strictly adhered to the DEVELOPMENT.md architectural boundaries.
• I have attached a screenshot or screen recording (if this touches the UI).
• I have updated the documentation accordingly (if adding a feature or changing a contract).

Verification

• Focused component regression: 8 tests passed.
• Focused Electron regression: passed; verifies the menu layer is above the backdrop, remains inside a 720×480 viewport, selects urgent, and persists the value.
• pnpm build: passed.
• pnpm pre-commit: not completely green due to an unrelated upstream baseline failure. The changed Task Editor E2E passed in the full run. Final E2E summary: 262 passed, 47 skipped, 1 flaky, 1 failed.
  • Stable failing baseline: tests/e2e/workspace-canvas.agent-status-watcher.spec.ts — opens a non-focused project notification at the corresponding agent expects viewport x/y movement greater than 30 but receives 0.
  • The failing test file is unchanged from upstream/main and is outside this PR's five-file diff. A focused rerun reproduces the same failure.
  • Submission with this disclosed exception was explicitly approved by the contributor.

📸 Screenshots / Visual Evidence

Task Editor priority menu at a constrained 720×480 viewport; the complete menu remains visible above the dialog backdrop. The verified local screenshot is pending direct upload because no browser-control session was available during submission.

[CLAassistant]

All committers have signed the CLA.