feat(webui): improve API error handling with structured toast notifications

[brancan]

Summary

This PR improves the WebUI's API error handling by adding a structured error display system on top of the existing toast notification component.

Changes

• New ShowAPIError(jqXHR, fallback) helper that parses common error
  shapes returned by the API (string, plain {detail}, Pydantic-style
  [{loc, msg, type}]) and renders them in a readable, multi-line toast.
• Enhanced ShowToast with:
  • Adaptive duration based on message length (longer messages stay
    visible longer instead of vanishing too quickly).
  • Optional HTML rendering for structured content.
  • Defensive coding around undefined forEach calls when responses are
    malformed.
• New CSS in customize.css for compact, readable error toasts
  (color-coded, monospace for codes/paths, line-broken for long messages).

Why

Today, when the API returns a Pydantic validation error or a structured
HTTP error, the WebUI either shows a generic message or dumps the raw
object to the console. End users have no signal of what's wrong, and
operators have to open the browser devtools.

This PR keeps the existing ShowToast API (no breaking change) and adds
a richer overlay path that:

1. Plays nicely with FastAPI's default error envelope.
2. Falls back gracefully for non-Pydantic errors.
3. Is opt-in: existing call sites keep working unchanged.

Backward compatibility

• No changes to existing ShowToast(message) callers.
• No new endpoints or schema changes.
• Pure frontend; the API layer is untouched.

Test plan

• Existing WebUI flows still show toasts as before.
• Trigger a Pydantic validation error (e.g. POST a malformed request
  to /libreapi/predefine/codec/{name}) and verify the new toast
  shows loc, msg and type clearly.
• Trigger a {detail: "..."} error and verify the toast shows the
  detail string.
• Trigger a network error (server down) and verify the fallback path.
• Run with a long error string and confirm the duration scales.

---

Cherry-picked from a downstream deployment of LibreSBC where this
patch has been running in production for several months. Original
commit: 82a0f62.