feat: ground unknown term summaries

[fly1d]

What changed

• Query the Chinese Wikipedia MediaWiki API when an unknown term is not covered by built-in knowledge.
• Turn the most relevant result into a sourced quick summary, three concepts, takeaways, and learning steps.
• Detect likely disambiguation pages and ask the user to choose among up to three meanings.
• Fall back to the existing local template after timeout, API failure, malformed data, or no useful result.
• Document the external term-query data flow and expose Wikipedia / CC BY-SA attribution on the source control.

Closes #6

Risk

• Low: copy, styles, documentation, or isolated cleanup
• Medium: interaction, API response, or shared behavior
• High: fetching, security boundary, deployment, or data compatibility

Verification

• npm run ci passes locally
• I tested the main user path affected by this change
• I checked desktop/mobile layout when UI changed
• I added or updated tests for behavior changes
• I documented anything intentionally not tested

Verification notes:

All 15 local checks pass. Real API and service-level smoke tests covered 量子纠缠 as a sourced result, Mercury as a dynamic clarification, and MCP as an unchanged built-in result. CI uses mocked MediaWiki responses and does not require external network access. The only UI change is source tooltip/accessibility metadata; layout is unchanged.

Review and release

• The diff is focused and contains no unrelated changes
• Error, empty, loading, and recovery states were considered
• Security and privacy impact was considered
• Rollback is understood

Rollback plan:

Revert this PR. Unknown terms will return to the existing local generic template; no data migration or dependency rollback is required.

[fly1d]

Code review completed with no blocking findings. Verified: built-in terms return before any external request; unknown terms use only the fixed Chinese Wikipedia endpoint with a 3.5-second timeout; relevance indices are sorted before selection; disambiguation, malformed data, failures, and empty results are covered; source attribution and the external data flow are documented; no dependency was added; all 15 checks pass.