feat(feedback): conversation downgrade notice + role requires model config

[btspoony]

Summary

User feedback round (2026-08-14) on dsh-llm-fallbacks — two product opinions:

1. 对话降级提示: when a fallback happens, the conversation must show the model downgrade. The existing fallbacks-switch chat node now says 模型已降级 / Model downgraded (was neutral "模型切换 / Model switch"), with a warn-tone title color via the existing --dsw-alias-state-warn-primary token. Summary unchanged: from → to (role · reason).

2. Role 必填 model 配置: a declared role without a model config (chain) is meaningless — the settings card now blocks save with a validation.roleChainRequired error (banner + inline hint mirroring the blank-row condition), and the host config validator warns (llm-fallbacks: role "<id>" has no model config — …), never crashes. Schema stays non-breaking (chain? optional); runtime defensive fallback (missing chain → rootChain / inherit-root) preserved.

Changes

• src/client/ConversationFallbackSwitch.tsx + locales.ts + css: downgrade-clear title (zh/en), warn alias token, JSDoc fix
• src/client/FallbacksCard.tsx: validateDraft role-chain-required (blocks save, incl. fallback: none + empty chain); inline hint when all chain selector rows are blank
• src/config.ts: validateFallbacksConfig warn for missing/empty role chain (names role id, no throw)
• docs/configuration.md, README.md, README.zh-CN.md: required-semantics + defensive-fallback notes
• Tests: +6 (conversation title zh/en/degraded; config warn ×3; card block+recover+blank-row); suite 409/409, build green

Verification

• SDD 3 tasks, per-task review APPROVE; plan QC tri 3× Approve (zero-residual; fix wave c9a8b36 → targeted re-review 3× Approve)
• QA (mandatory): PASS-WITH-NOTES — zero blocking defects; browser screenshots + real-session startup warn pending-user (sandbox cannot run real dsh web; same split as prior rounds)