fix(llm-pi-ai): let a model declare the request modalities it accepts

A model the installed pi-ai catalog does not describe was reported as
text-only with no way to say otherwise, so a vision model added through
the custom-provider form was refused at every image admission point.
The justification in the source described the DeepSeek chat-completions
serializer, which does reject image blocks; the pi-ai request converter
and every wire protocol it speaks carry images.

Modalities now resolve entry `input` -> installed catalog entry -> route
`defaultInput`, the chain the two capacity fallbacks already use, so the
route value is a fallback and never narrows a catalog model. Its default
stays `[text]`: nothing can interrogate a gateway for its modalities, and
over-claiming admits an image the provider rejects mid-turn, after prompt
admission has already committed the message.

docs/user/guide/providers.md

@@ -28,6 +28,57 @@ The Provider ID is permanent because requests, saved sessions, model defaults, a
 
 Under **Model catalog**, choose **Fetch available models** to query the base URL and credential currently shown in the form. Selecting candidates updates the draft; the provider is not stored until you save. Catalog providers use their installed catalog without a network request.
 
+### Image input
+
+A model you enter by hand is treated as text-only until it says otherwise, because nothing can ask an endpoint which modalities it accepts. Attaching an image to such a model is refused before it is sent, naming the model.
+
+A vision model on a custom provider therefore needs one line. The form has no field for it; add `input` to the model in `$DSH_HOME/settings.yaml`:
+
+```yaml
+llm-pi-ai:
+  providers:
+    my-gateway:
+      apiKeyEnv: GATEWAY_API_KEY
+      api: openai-completions
+      baseURL: https://gateway.example/v1
+      models:
+        - id: legacy-chat
+        - id: vision-preview
+          input: [text, image]
+```
+
+`input` accepts `text` and `image`, and applies to that model alone, so one route can serve both kinds. Omitting it — or writing an empty list, which means the same thing — keeps whatever the installed catalog records for that model, and falls back to the route's `defaultInput` for a model the catalog does not describe.
+
+If every model you entered by hand takes images, set the fallback once on the route instead of on each of them:
+
+```yaml
+llm-pi-ai:
+  providers:
+    vision-gateway:
+      apiKeyEnv: GATEWAY_API_KEY
+      api: openai-completions
+      baseURL: https://vision.example/v1
+      defaultInput: [text, image]
+      models:
+        - id: first-model
+        - id: second-model
+```
+
+`defaultInput` is a fallback, not an override, and defaults to `[text]`: on a catalog provider it answers only for models the catalog does not describe, so it never removes images from a catalog model that has them. Narrow one of those with that model's own `input`. A catalog provider has no `models` list to put it in, so write it under `modelOverrides`, keyed by model id:
+
+```yaml
+llm-pi-ai:
+  providers:
+    anthropic:
+      modelOverrides:
+        claude-sonnet-4-5:
+          input: [text]
+```
+
+Every list must name at least one modality except a model's own, where an empty list means the same as omitting it. An unknown modality is refused wherever it is written.
+
+Both fields state a claim about your endpoint rather than checking it. A model that declares images its endpoint does not serve is not caught here; the provider rejects the request instead.
+
 ## Select a model
 
 Configured providers appear in the model picker. Selecting a model also makes it the default for new sessions. A session that has already sent a request retains the model recorded in its own log.
@@ -39,6 +90,8 @@ If a saved default names a provider that was deleted, the composer displays **Se
 - **`MISSING_CREDENTIAL`** — Store the provider key through the Models page or supply the referenced environment variable.
 - **`UNKNOWN_MODEL`** — Select a configured model or add the missing model to the custom provider.
 - **Fetching available models returns 401** — Check the key. Model discovery calls the OpenAI-compatible `GET /models` endpoint; enter models manually for endpoints that do not provide it.
+- **An image is refused before sending** — The model declares no image modality. Give a custom provider's model `input: [text, image]`; DeepSeek's own chat-completions route is text-only and cannot be configured otherwise.
+- **The provider rejects a request carrying an image** — The model declares images its endpoint does not actually serve. Remove `image` from whichever list granted it — the model's `input`, or the route's `defaultInput` — then start a new session: the attached image stays in the session log, so the same request repeats until the session moves off it.
 
 ## Advanced configuration