Merge remote-tracking branch 'origin/master' into worktree/default-model-persistence

Carries two edits beyond conflict resolution, both forced by what master
brought in:

- `CustomProviderCard`: master added front-end key validation and a
  component-level `keyValue` (already trimmed) while still writing
  `apiKeyEnv` unconditionally. Kept this branch's blank-key rule and its
  committed-profile retry gate, and adopted master's single `keyValue` so
  the component has one spelling of the key rather than two.
- `docs/user/guide/providers`: master merged #1810, whose default-model
  section still taught overriding the `api-gateway` row in
  `$DSH_HOME/config.yaml` — the behavior this branch replaced. Rewritten
  for the settings section the picker now writes, plus the review fix from
  #1810 replacing the colloquial 挂着 in the opener.

packages/client/ui-models/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/client/ui-models/README.md
-README.md: fe8e9851978ebd900fa43eb28f52a5a803501b63
-README.zh.md: c11ec125c6bf896c32db74161a263ee2b73c88be
+README.md: 1d9c98dfd1e0cec0fa4cb33df9ffe2640be8be05
+README.zh.md: d3437c6f13be49ea73d6b3a51bee664b32521e01