feat(ui): add strict foreground action registry

[MADPANDA3D]

Summary

Adds a strict browser-owned registry for three fixed foreground actions: open Calendar, close Document, and minimize Document. The v1 envelope is versioned, capped at 1 KiB, requires an empty payload, and rejects unknown fields, targets, selectors, URLs, markup, and executable text.

This PR adds only the host seam and existing-module handlers. It does not add a server emitter, Voice Orb UI, client-state reporting, or a plugin compatibility promise. Existing legacy UI-control behavior remains unchanged.

Target branch

• This PR targets dev, not main.

Linked Issue

Part of #5549. Related to #4118 and discussion #4439.

Type of Change

• Bug fix (non-breaking — fixes a confirmed issue)
• New feature (non-breaking — adds new behaviour)
• Breaking change (changes or removes existing behaviour)
• Refactor / cleanup (behaviour unchanged)
• Documentation only
• CI / tooling / configuration

Checklist

• I searched open issues and pull requests; no existing strict foreground-action registry duplicates this proposal.
• This PR targets dev.
• The registry exposes only fixed core-owned action IDs with exact empty v1 payloads.
• Server-side authorization is explicitly documented as required before event emission.
• The new static module is included in the service-worker precache with a cache-version bump.

How to Test

1. Run: python -m pytest -q tests/test_foreground_actions_js.py
2. Run Node syntax checks for static/js/foregroundActions.js, static/js/chatStream.js, and static/app.js.
3. Start the app and send the exact foreground_action envelopes documented in docs/foreground-actions.md.
4. Confirm Calendar opens/restores, Document minimizes/closes, and a selector-bearing or unknown action fails closed.

Local validation covered 22 targeted tests plus Ruff, formatting, Node syntax, Python compilation, and diff checks. Live FastAPI + Playwright passes exercised all three actions, verified the service-worker cache contained the module, rejected an unknown selector-bearing action with unknown_action, and observed zero browser console errors. The minimized-document close path also cleared its persisted restore marker and remained closed after reload.

Visual / UI changes

No new visible component or styling is introduced. The actions operate existing Calendar and Document surfaces.

• Screenshot or short clip attached — no new pixels or styling are added.
• Style match — existing surfaces are reused.
• No new component pattern — this is a bounded host contract only.
• I am not an LLM agent submitting a bulk PR — this contribution is coding-agent assisted, disclosed in Proposal: bounded in-tree host seams for voice and UI composition #5549, and intentionally split into an independent seam.