ui: Bigtrace UI support for local servers and a server picker

The Bigtrace UI changes for the local Bigtrace server, split out so they
can land ahead of the server itself. They only touch ui/ and are inert
against today's backends: everything new is enabled by what the
connected server advertises.

- Server picker (connection button): the built-in servers (the Bigtrace
  service and `Local server` on http://127.0.0.1:5001/v1) and servers
  the user adds, each with whether it answers (`GET /`, falling back to
  `GET /trace_presets`). Picking one sets the endpoint and reloads;
  `?server=` still works. The endpoint field stays, as before: type any
  URL and `Reload to apply`, or `Reset` to the default; an endpoint that
  isn't listed shows as "Current server", which can be added to the
  list. Added servers can be edited (name and URL; the active one
  follows) and removed. A plain-http server that is
  not loopback is refused from an https page, with a pointer to SSH
  tunnelling. Built from the stock widgets (Popup, Form/FormLabel,
  TextInput, Button with tooltips, Icon, Callout, CodeSnippet, Spinner),
  laid out like the extension server list; the server in use has a
  check mark, as menus mark their selection.
- `Local server` > `Set up` shows the install and tunnel commands
  (adding `--cors-origin` for this UI's origin when needed) and switches
  to the server once it answers. A readable `Origin not allowed` error
  from the server becomes the command that allows it.
- Multi-query tabs: several queries per tab run in one pass ("Add
  Query"), each into its own result table; tables are named, with
  overwrite confirmation.
- A PerfettoSQL / DuckDB SQL RadioGroup in the topbar, and Add Query,
  shown only when the server advertises the `duckdb` and `multi_query`
  features from `GET /` (only the local server does). A DuckDB or
  multi-query tab restored on a server without them says why it can't
  run (with a way to a PerfettoSQL tab) instead of failing, and new tabs
  only inherit an engine the server runs.
- Polling and status follow every query of a pass; history items show
  the engine and result table.
- `ui/build --bigtrace` dev servers proxy /v1 and /trace to a local
  server ($BIGTRACE_SERVER_PORT, default 5001).
- zod runs jitless in the Bigtrace bundle: its CSP has no
  'unsafe-eval'.

Tests: Vitest (server_list, server_capabilities, endpoint_storage,
polling_controller, query_tabs_state, bigtrace_query_client); 294/294
in src/bigtrace pass. Checked by hand against a local Bigtrace server.