…ixes

User-facing:
- Gen UI dropdown label drops the 'v1-compatible' suffix → just 'A2UI'.
- Sidebar mode auto-opens the chat panel on entry (was: 'click the
  launcher first'). Hint text updated for the new flow.
- All three modes (embed, popup, sidebar) switch from
  `:host { height: 100% }` to `:host { display: block; flex: 1;
  min-height: 0 }` so the chat surface sits below the toolbar via
  flex column, not overlapping it.

E2E fallout from PR #444's toolbar tighten:
- test-helpers' toolbarSelect/selectToolbarOption now identifies fields
  via `[data-field="<label>"]` (added to demo-shell.component.html);
  the prior `filter({ hasText })` broke when the per-field labels were
  removed.
- lifecycle 'new conversation clears local thread' test updated to use
  the sidenav's 'New chat' (the toolbar 'New conversation' was removed
  in #444). Asserts a fresh thread id is created server-side.
- debug-devtools 'sidebar launcher remains reachable' now closes the
  auto-opened panel first so the launcher is the affordance under test.
- keyboard-accessibility 'Escape closes ... sidebar' no longer clicks
  the launcher (hidden when panel is auto-open); asserts the panel is
  already open, then Escape closes it.
- control-palette's Gen UI assertion follows the new 'A2UI' label.

…n + tighten

PR #444 (toolbar tighten) and the auto-open sidebar landed several e2e
test fallouts that surfaced on CI:
- model-picker queried .demo-shell__field via hasText filter; updated
  to use the data-field attribute introduced for the toolbar.
- mode-routing 'landmarks' uses Escape to dismiss the auto-opened
  sidebar panel (close-button click was hanging against the overlapping
  debug-devtools panel in CI).
- mode-routing 'cross-mode persistence' no longer clicks the launcher
  before asserting the conversation in /sidebar — panel is open.
- debug-devtools 'sidebar surface remains reachable' updated to verify
  the auto-opened panel + its close button stay visible while
  chat-debug is open (was: click the now-hidden launcher).
- demo-shell toolbar adds flex-wrap so small viewports (480px) don't
  overflow horizontally — the previous overflow-x:auto would clip the
  chat-select popover menus, so wrapping is preferred.