Map structured errors to operator copy via a WS banner

[AquiGorka]

Consumer side of the coordinated StructuredError adoption (pairs with network-dashboard-platform#13). The dashboard's only backend link is the network WebSocket, so structured errors ride it too — the "structured-error path" here is the WS payload, not an HTTP body. Two error sources, both surfaced through one shape-agnostic mapper:

1. Transport errors (connect fail / drop / reconnect) — previously a silent console.warn plus a hardcoded generic banner that wasn't even wired up.
2. { type: "error" } frames the backend now pushes over the socket (e.g. a failed council-platform topology refresh) — the connection is still live but some data may be degraded.

What changed

• lib/error-copy.ts (new): errorCopy() mirroring moonlight-pay's friendlyError — known code → copy, else a safe server message (guarded by isSafeSentence), else a generic fallback. Reads .code regardless of which backend produced it, so a council-platform code bubbled through network-dashboard-platform maps the same as a native one.
• network-events: adds the StructuredError type + the additive { type: "error" } frame; parseServerFrame validates it (mirrors the backend, no subprotocol bump).
• ws-client: onError handler; onmessage switches over frame type.
• dom.renderError: now displays the (mapped) message instead of the old hardcoded string, keeping the generic sentence only as an empty-message fallback.
• app.ts: a connection / degraded-data banner wired to both transport status (closed/not-configured → mapped copy; cleared on open) and backend error frames, all through the mapper.

No new REST path / CSP / config changes — errors ride the existing WebSocket. Council-platform failures reach the SPA via the backend's TOPOLOGY_REFRESH_FAILED frame; the mapper stays shape-agnostic so any upstream code maps.

Tests

deno check / lint / fmt / dev+prod build / test green. New: errorCopy mapper (known/unknown code, safe-message passthrough, unsafe-message rejection, transport codes, code-over-message), error-frame parsing, and renderError now-displays-mapped-copy + fallback. 25 tests pass.

Version bump 0.2.15 → 0.2.16 (own commit).

Merge order

Merge after network-dashboard-platform#13 (platform anchors the wire contract).