frontend: add WarnBanner Preact component, compose ApiErrorPanel from it

mara, on review: "i want each component to import its own css file
itself[;] if you need a bunch of extra css externally, its not a proper
component ... you may need to migrate other components that you would
want to use first".

WarnBanner is the Preact-component successor to the shadow-DOM
<hive-warn> custom element (same three-tier info/warning/error visual
language, colours copied faithfully from hive-warn.css). ApiErrorPanel
now composes it instead of owning a copy of the border/colour/pulse
rules itself — its own CSS is back down to just the layout that's
actually specific to it (heading, copy button, detail text).

Re-verified with a fresh mock-server screenshot: same rendered output as
before, now via composition instead of a duplicated banner shape.

frontend/packages/shared/package.json

@@ -34,7 +34,9 @@
     "./jobq-rollup.css": "./src/jobq-rollup/jobq-rollup.css",
     "./api-error.js": "./src/api-error.ts",
     "./api-error-panel.js": "./src/api-error-panel/ApiErrorPanel.tsx",
-    "./api-error-panel.css": "./src/api-error-panel/api-error-panel.css"
+    "./api-error-panel.css": "./src/api-error-panel/api-error-panel.css",
+    "./warn-banner.js": "./src/warn-banner/WarnBanner.tsx",
+    "./warn-banner.css": "./src/warn-banner/WarnBanner.css"
   },
   "files": [
     "src/"