fix(desktop): add approval dialog fallback when inline controls are absent

[konsisumer]

What changed and why

Per the reporter's 2026-06-05 follow-up on #37812, Desktop can still reach a state where a command approval request times out with no notification, dialog, inline prompt, or visible affordance, even after the merged event-routing and grouped-tool fixes in #38578 and #38829. This adds a guarded modal fallback for approval.request: the existing inline tool-row approval remains the primary UX, but if no inline approval control is mounted for the active session, PromptOverlays now renders a command approval dialog with Run once / Allow this session / Reject actions.

The inline approval component now registers its active session while mounted, and the prompt store exposes active-session inline visibility so the fallback does not duplicate the normal inline controls. New regression coverage verifies the fallback appears when no inline bar exists, stays hidden while an inline bar is mounted, sends approval.respond, and tracks inline mount state.

How to test

• npm run test:ui -- src/components/prompt-overlays.test.tsx src/components/assistant-ui/tool-approval.test.tsx src/components/assistant-ui/tool-approval-group.test.tsx src/store/prompts.test.ts (blocked locally: vitest is not installed in this checkout)
• npm run type-check --workspace apps/desktop (blocked locally: tsc is not installed in this checkout)
• /opt/homebrew/bin/timeout -k 30 480 sh -c 'pytest tests/ -q -x --timeout=60 "$@"' sh (blocked locally during collection: missing Python dependency fastapi)

What platforms tested on

• Not runtime-tested. Local validation was limited to git diff --check because this sandbox lacks the Desktop Node toolchain and the Python environment is missing fastapi.

Fixes #37812

[konsisumer]

Closing: referenced_issue_closed. all referenced issues closed: [37812]