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 @@
 </head>
 
 <body>
-  <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>
   <!--
       This HTML file is a template.
       If you open it directly in the browser, you will see an empty page.