fix(desktop): hide docked native views under the extension prompt

[vastsa]

Same occlusion as #879 (fixed for the project dialogs in #1378), still present for the
prompt a trusted extension raises.

Symptom

With a docked native view in the work panel (work-panel browser, file manager, …),
ui.confirm / ui.select / ui.input prompts from a trusted extension rendered
under the WebContentsView: the modal and its backdrop were covered on the right
side and the panel kept taking input.

Root cause

ExtensionPromptDialog declared only useHostSafetySurface(), which makes the
renderer-side plugin layers step aside. It never registered useBlockingOverlay(),
the only mechanism that hides a native view (WorkPanel → PluginViewTab blocked →
pluginViewSetVisible(false)).

Change

• Register useBlockingOverlay() in ExtensionPromptDialog. The dialog mounts only
  while a prompt is queued, so mounting is exactly the window that must block the
  panel — no wrapper split or prop needed.
• Extend host-safety-layer.test.mjs with a source assertion for the registration.
• Extend the mounted dialog fixture (dialog-overflow) to assert the native preview is
  suppressed while the prompt is up and released after it closes.

Why only this dialog

I audited every other surface that renders role="dialog" / aria-modal without the
hook:

• Settings/plugins/scheduled dialogs (ProviderSetupDialog, VendorAccountDialog,
  OAuthLoginDialog, SkillEditorSheet, SubagentEditorSheet, McpEditorSheet,
  McpMarketPanel, SkillMarketPanel, prompt-enhancement-card, ReleaseNotesDialog):
  those pages hide the work panel (useAppShellRuntime → pageHidesWorkPanel), so the
  dock and its native view are unmounted before the modal opens.
• SearchDialog: already equivalent — AppShell passes panelBlocked={searchOpen} to
  WorkPanel, which hides the same native view.
• NotificationCenter, SessionHoverCard, ContextUsageInspector, ScopeControl,
  ScheduledWeekdaySelect, LiveVoiceControls, LiveVoiceDetails: anchored popovers
  that do not overlay the dock (ContextUsageInspector even clamps to .main-pane).
• StartupRecovery: only up before the shell mounts.

Verification

• node --test test/*.test.mjs: 3488 pass, 0 fail.
• node scripts/test-dialog-overflow.mjs (real Electron + Chromium, mounted production
  dialogs): 41/41 pass, including the two new checks.
• Negative control: commenting out the registration makes
  extension prompt suppresses native preview while open fail (40/41), so the new
  check really guards the fix.
• tsc -p apps/desktop/tsconfig.json --noEmit: clean.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.