#2546 web: the closed Sampling drawer leaves the keyboard's Tab order; opening it focuses it, closing returns focus

Descripción

**What it changes.** Closed, the Chat page's Sampling drawer is only moved off-screen (`transform: translateX(100%)`), so its 14 controls (Close, the four Thinking buttons, temperature, top-p, top-k, max tokens, seed, the switches, Reset, Apply) stayed in the keyboard's Tab order right after **Send**, inside `aria-hidden="true"`. A keyboard user tabbed through invisible buttons, and a screen reader landed on controls it was told were hidden. Now:

- the drawer is `inert` while closed (in `index.html` at load, and set in `openDrawer`);
- opening it moves focus to its **Close** button (before, focus stayed on the button behind the scrim);
- closing it (Escape, Close, Apply, a click on the scrim) puts focus back on the **Sampling** button (before, after Escape focus stayed on a button that was now off-screen).

**What it leaves alone.** The drawer's look, its settings and what they send, and every other part of the page. A browser without `inert` (Chrome < 102, Firefox < 112, Safari < 15.5) behaves as before, plus the focus moves.

**How it was checked.**
- Headless Chromium (Playwright) against a running Strata 0.1.44.1 server (RTX 5070 Ti, Linux), with this branch's `index.html` and `app.js` served in place of the release's, Tab pressed 30 times from the message box:
  - before: 14 drawer controls reached with the drawer closed; after opening, focus on `sampling-btn`; after Escape, focus on a button inside the closed drawer;
  - after: 0 drawer controls reached; after opening, focus on `drawer-close` and Tab moves through the drawer; after Escape, focus on `sampling-btn`.
- New static test `WebApp.test_the_closed_drawer_is_out_of_the_tab_order` (fails without the change). `python -m unittest serve.test_server serve.test_security serve.test_frontend serve.test_request_hardening serve.test_monitor serve.test_runconfig`: all pass (392 tests).

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