refactor(frontend): use ApiError.toUserMessage() instead of 'Network error' fallback

[shiba4life]

Summary

Stops ~28 ad-hoc `'Network error'` string fallbacks across the React UI from mis-labelling HTTP 4xx/5xx responses as network failures. Routes everything through `ApiError.toUserMessage()` so the user sees the actual server message (e.g. a 404 body's "schema not found") and only sees "Network connection failed" when the request really failed at the network layer.

• Tightens `toErrorMessage()` in `src/utils/schemaUtils.ts` to defer to `ApiError.toUserMessage()` for `ApiError` instances.
• Tightens `ApiError.toUserMessage()` to prefer `this.message` (the server-supplied descriptive text — including the structured 400/404 payload bodies wired up in feat(query): 400 on unknown field names in POST /api/query #1003 / feat(mutation): 400 on unknown field names in POST /api/mutation #1007) over the generic constants. Synthetic `HTTP ` placeholders are filtered out so we don't surface that.
• Rewrites 28 catch-block fallbacks across 20 components — replaces `'Network error'` with operation-specific copy (`'Search failed'`, `'Failed to load X'`, etc.) for the rare case the helper returns empty.
• `StatusSection.tsx` keeps the `Network error: ...` prefix but now gates it on `isNetworkError(error)` so it only fires for real network failures.

Out of scope

• `QueryTab.tsx:106` — sister task 039b3 owns the rich error-UI rewire for that path.
• Settings panels (`DatabaseSettings.tsx`, `BackupSettingsPanel.tsx`) — different copy pattern (`Network error: ${msg}` template) that was not part of the scope.
• `Network error while ...` strings in some persona panels — different, more descriptive strings; the DoD only required the literal `'Network error'` fallback to go.

Test plan

• `npm run typecheck` — clean.
• `npm test` — 788 passing, 6 skipped (no regressions).
• `grep -rn "'Network error'" src/server/static-react/src/components` — zero hits.
• Manual: trigger a 404 from `/api/schema/{name}` (DataBrowserTab) and confirm the server message surfaces.

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