fix: defer 1.2 GB model preload until sidebar opens (closes #116) - #205

Summary

The admin-sidebar bundle loads on every wp-admin page (intentional — the toggle lives in the admin bar). But mounting `AdminSidebar.jsx` immediately ran `initializeApp()`, which fired the WebGPU check, looked up saved provider preferences, and started downloading / loading the ~1.2 GB Qwen 3 1.7B model into GPU memory.

For users who rarely open the sidebar (most page loads), that's a huge unwanted cost — both memory and the visible download progress bar in the admin bar.

Net: +40 / -2 lines, 2 files touched.

Fix

React tree still mounts on every admin page (cheap — just a few KB of components), but the WebGPU/model initialization is gated on a `hasOpened` state that flips when the user clicks the toggle for the first time.

Wiring

• `admin-sidebar.js` dispatches a custom `wp-agentic-admin/sidebar-opened` event when the toggle adds the `is-open` class. Subsequent toggles still fire the event (harmless — `hasOpened` is already true).
• `AdminSidebar.jsx` subscribes to the event, sets `hasOpened`, and a second useEffect that depends on `hasOpened` runs `initializeApp` when the flag flips.
• Initial `hasOpened` reads `is-open` from the DOM, so if some upstream change makes the sidebar start open, we still initialize on first render.
• Listener is removed once `hasOpened` becomes true (no leak).

Memory impact

Verification

• `npm test` — 96 passing, unchanged
• `npm run build` — clean
• `composer lint` — clean
• `npx wp-scripts lint-js` — clean

Manual test plan

• Open Playground at `/wp-admin` (any page that's NOT the plugin's own admin page)
• Confirm: no model loading happens — ModelStatus widget on the admin bar should be idle / not progress
• Click the superhero icon to open the sidebar
• Confirm: WebGPU check fires, model loads (you'll see the progress bar / status)
• Close + reopen sidebar — no second load triggered

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