feat(ai): make lookup language explicit and deterministic (#286)

[yicheng47]

Closes #286.

Problem

Look Up's main definition/context did not deterministically follow the configured Lookup language:

1. English emitted no language directive — the "Respond entirely in {X}" instruction was only built when language != "en", so a Chinese selection with Lookup language = English drifted back to Chinese. (Core bug.)
2. Source-language lookup was implicit — "Chinese definition for a Chinese book" only happened as a side effect of defaults/bug feat: frontend static #1, so it read as a glitch rather than a choice.

Changes

• src-tauri/src/commands/ai.rs
  • Add LOOKUP_LANGUAGE_SELECTION sentinel + main_language_clause() helper.
  • lookup_system_prompt now always emits a "Respond entirely in {clause}" directive for both definition and context (drops the language != "en" guard). The brief-translation gloss stays an independent add-on, allowed alongside "Same as selection".
  • explain_system_prompt uses the shared language_name map, handles "selection" ("the same language as the selected passage"), and emits the English directive explicitly.
  • lookup_language falls back to "selection" (instead of the app language) in both ai_lookup and ai_explain.
• src/components/settings/ToolsSettings.tsx — prepend a Same as selection option to the Lookup-language dropdown only (gloss/explain/translate keep concrete languages); default the dropdown to lookup_language || "selection"; widen the select to fit the label.
• i18n — add settings.tools.sameAsSelection ("Same as selection" / "跟随所选内容").

No setting keys renamed, no migration: "selection" is a value the existing lookup_language key can hold. Existing installs that never set lookup_language shift from app-language to source-language definitions — intentional and now self-documenting in Settings.

Tests / checks

• cargo test — full suite green; new prompt regressions: lookup_english_emits_explicit_english_directive, lookup_selection_uses_source_language, lookup_selection_allows_gloss, explain_selection_uses_source_language (plus updated existing tests).
• npm run lint — 0 errors (pre-existing warnings only); tsc --noEmit — clean.
• Manual smoke test of the Settings UI passed.

🤖 Generated with Claude Code

[yicheng47]

Review findings:

No remaining must-fix issues found. The PR matches issue #286: lookup/explain prompts now emit explicit response-language directives including English, the selection sentinel is handled without leaking as a literal target language, backend defaults align with the new source-language mode, and the Tools settings dropdown exposes “Same as selection” only for Lookup while keeping gloss/explain/translate targets concrete.

Verification:

• gh pr diff 287 reviewed.
• gh pr checks 287 passed: Frontend, Backend.
• Local checks previously rerun on the same diff: cargo test lookup, cargo test explain_prompt, pnpm exec tsc --noEmit, pnpm run lint (0 errors, existing warnings), git diff --check.