feat: expose the active search source in search settings (#109)

[RealAlexandreAI]

Summary

Implements the UI half of #109: make the actual search source explicit on the search settings page so users can verify configuration and troubleshoot search failures.

• No third-party provider configured → info alert: built-in search is used (no API key required; stability not guaranteed), and it switches automatically once a provider is configured.
• Provider configured → success alert showing the active provider name (e.g. "当前搜索源：Tavily"), noting built-in search is no longer loaded by default to avoid exposing multiple search tools to the model at once.

Scope note

This covers the UI/UX part of #109 (explicit search-source display). The tool-selection/mutual-exclusion logic lives in the harness agent dependency, so it is intentionally out of scope here — the alert copy already states the intended behavior (built-in search not loaded when a third-party provider is configured).

Test plan

• `vitest run` — 177 passed (39 files, +1 new test file)
• `tsc --noEmit` — clean
• zh/en locale keys added consistently (`advancedSettings.search.sourceBuiltin*`, `sourceConfigured*`); verified no key drift between locales
• New tests: built-in hint shown when no provider configured; configured provider shown as current source

CHANGELOG.md updated.