feat: ground source follow-up answers

[fly1d]

What changed

• Preserve up to six source excerpts, capped at 280 characters each, in Wikipedia-backed analysis results.
• Answer application, example, history, mechanism, limitation, explanation, and deeper-background questions from matching excerpts.
• Explicitly state when the available source cannot answer a question instead of filling the gap with generic or invented facts.
• Show source-appropriate quick questions for encyclopedia topics while keeping built-in topic actions unchanged.

Closes #8

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 16 local checks pass. A real service test used a live 量子纠缠 result as /api/ask context and verified that unsupported application/price questions did not invent facts. Browser testing at 1280px and 390x844 verified the three encyclopedia quick questions, the 再讲详细 interaction, no text or horizontal overflow, and no console warnings/errors.

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. Wikipedia summaries remain available, while follow-up questions return to the existing generic rules; no migration or dependency rollback is required.

[fly1d]

Code review completed with no blocking findings. Verified: source excerpts are bounded at generation and consumption; source-aware follow-ups run before generic rules; unsupported questions return an explicit evidence-limit response; built-in MCP follow-ups remain unchanged; encyclopedia-only quick questions preserve button count and layout; desktop/mobile browser checks and all 16 automated tests pass.