Vision: attach an image with a prompt and have the model read it (#129)

## Summary
- Closes #128.
- Chat composer accepts PNG, JPEG, WebP, and GIF along with existing documents.
- Selected image shows a thumbnail before send. Filename stays hidden for images. Documents still show the filename.
- User turn keeps the image after reload, from the prompt `image` field the backend returns.
- Oversized or unsupported images show the websocket error on the assistant bubble.
- Landing copy and `llms.txt` no longer list vision as roadmap-only.

Needs the matching `chat_backend` branch `feat/128-vision-image-prompt` so the model actually reads the image.

## Test plan
- [ ] Attach a PNG with a prompt that asks to read text in the image. Answer uses the picture.
- [ ] Thumbnail shows before send, with no filename.
- [ ] Reload the conversation. The image is still on the user turn.
- [ ] CSV/PDF attach still works.
- [ ] Text-only prompt unchanged.
- [ ] A BMP or an image over 8 MB shows an error instead of a silent failure.

Reviewed-on: #129

llm-fe/public/index.html

@@ -83,7 +83,7 @@
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-  <div id="root"><h1>Hesychia is a calm AI assistant.</h1><div id="crawler-prose"><p>Quiet, focused AI chat — private by default — is how this Hesychia product is built.</p><p>It remembers what you told it and your documents, so you never start over, and conversation memory is a setting you control.</p><p>We store your account, your conversations, and the documents you upload.</p><p>We do not sell your data, and we do not use advertising cookies.</p><p>Canceling does not delete documents, and they stay until you delete them or delete the account, and account deletion is a soft-delete.</p><p>It remembers what you told it and your documents, so you never start over.</p><p>Your documents stay stored and searchable, and answers are grounded in your own files.</p><p>Business accounts are simple management for your team, and company managers can manage team accounts.</p><p>Early adopters pay $10 per month, locked for the lifetime of the account, and that price never goes up.</p><p>Image generation is on the roadmap, so a prompt can create a picture inside Hesychia.</p><p>Vision is on the roadmap, so you can ask questions about an image you upload.</p></div><nav aria-label="Related pages"><a href="/pricing/">Hesychia pricing — $10/month, no ads</a><a href="/privacy/">What Hesychia stores and does not sell</a><a href="/changelog/">Dated Hesychia product updates</a><a href="/terms_of_service/">Hesychia terms of service</a><a href="/account-deletion/">How to request Hesychia account deletion</a></nav></div>
+  <div id="root"><h1>Hesychia is a calm AI assistant.</h1><div id="crawler-prose"><p>Quiet, focused AI chat — private by default — is how this Hesychia product is built.</p><p>It remembers what you told it and your documents, so you never start over, and conversation memory is a setting you control.</p><p>We store your account, your conversations, and the documents you upload.</p><p>We do not sell your data, and we do not use advertising cookies.</p><p>Canceling does not delete documents, and they stay until you delete them or delete the account, and account deletion is a soft-delete.</p><p>It remembers what you told it and your documents, so you never start over.</p><p>Your documents stay stored and searchable, and answers are grounded in your own files.</p><p>Business accounts are simple management for your team, and company managers can manage team accounts.</p><p>Early adopters pay $10 per month, locked for the lifetime of the account, and that price never goes up.</p><p>Image generation is on the roadmap, so a prompt can create a picture inside Hesychia.</p><p>Upload an image with your prompt and Hesychia reads the picture, so you can ask about what is in it.</p></div><nav aria-label="Related pages"><a href="/pricing/">Hesychia pricing — $10/month, no ads</a><a href="/privacy/">What Hesychia stores and does not sell</a><a href="/changelog/">Dated Hesychia product updates</a><a href="/terms_of_service/">Hesychia terms of service</a><a href="/account-deletion/">How to request Hesychia account deletion</a></nav></div>
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llm-fe/public/llms.txt

@@ -2,7 +2,7 @@
 
 > Hesychia is a calm AI assistant with quiet, focused AI chat that stays private by default.
 
-Hesychia is a $10/month AI chat product from AI ML Operations, LLC. Early adopters pay $10 per month, locked for the lifetime of the account, and that price never goes up. It stores your documents so answers can use your files, and company managers can manage team accounts. Conversation memory is a setting you control, and canceling a subscription does not delete documents. Account deletion is a soft-delete, and stored account data may remain. Image generation and vision are on the roadmap.
+Hesychia is a $10/month AI chat product from AI ML Operations, LLC. Early adopters pay $10 per month, locked for the lifetime of the account, and that price never goes up. It stores your documents so answers can use your files, and company managers can manage team accounts. Conversation memory is a setting you control, and canceling a subscription does not delete documents. Account deletion is a soft-delete, and stored account data may remain. Image generation is on the roadmap. Upload an image with a prompt and Hesychia reads the picture.
 
 ## Pages