fix(i18n): map expected failure codes instead of raw messages

[orangeCatDeveloper]

Summary

Five renderer sites still rendered a raw error.message or decided what to show by sniffing for CJK characters, so English users saw operator/Host prose verbatim (or, where sniffed, lost the information). Each is now code → catalog:

• Runtime Host management dialog: the operator CLI's service-management frame keeps error.code as an open string on the wire (operator version skew), so the closed union lives at the presenter — RuntimeHostManagementErrorCode (26 codes) mapped per locale in settings-projects-copy.ts with an Object.hasOwn unknown fallback; all five response.error.message renders go through it. A test reads the CLI's RuntimeHostServiceManagerError code union from source and asserts every code is mapped, so a new CLI code fails the desktop test suite instead of silently falling back.
• Thread search: error.reason was already a typed SearchErrorReason; the five reasons thread search emits are mapped in shell-controls-copy.ts, the rest fall back, and the thrownErrorMessage seam that rendered error.message is gone (raw error goes to console.error).
• provider-panel-shared.ts and artifact-pane.tsx: the CJK-passthrough guards were dead — the main handlers they defend against throw English only today — so they are deleted with a guard test.

Not changed: skill-status.ts sniffs skill.description, which is third-party SKILL.md data rather than our copy; the right fix is to show data as-is and drop the keyword blurbs, a product decision left for a separate discussion.

Refs #2672

Verification

workspace typecheck:                    0 errors
desktop main tests (dist):              1997 pass / 0 fail (3 new: code map completeness vs CLI source, zh+en render, unknown fallback)
packages/ui tests:                      335 pass / 0 fail
knip (apps/desktop, packages/ui):       clean
renderer architecture check:            passed against origin/main
format:check / biome:                   clean

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Tool(s) and scope: Claude Code — producer tracing, implementation, tests, and this description, under the contributor's direction; the commit carries a Generated-by: Claude Code trailer.

Checklist

• Tests cover the change and fail without it