webui: remove client-side context pre-check and rely on backend for limits (#16506)

webui: remove client-side context pre-check and rely on backend for limits (#16506)

* fix: make SSE client robust to premature [DONE] in agentic proxy chains

* webui: remove client-side context pre-check and rely on backend for limits

Removed the client-side context window pre-check and now simply sends messages
while keeping the dialog imports limited to core components, eliminating the
maximum context alert path

Simplified streaming and non-streaming chat error handling to surface a generic
'No response received from server' error whenever the backend returns no content

Removed the obsolete maxContextError plumbing from the chat store so state
management now focuses on the core message flow without special context-limit cases

* webui: cosmetic rename of error messages

* Update tools/server/webui/src/lib/stores/chat.svelte.ts

Co-authored-by: Aleksander Grygier <redacted>
* Update tools/server/webui/src/lib/stores/chat.svelte.ts

Co-authored-by: Aleksander Grygier <redacted>
* Update tools/server/webui/src/lib/components/app/chat/ChatScreen/ChatScreen.svelte

Co-authored-by: Aleksander Grygier <redacted>
* Update tools/server/webui/src/lib/components/app/chat/ChatScreen/ChatScreen.svelte

Co-authored-by: Aleksander Grygier <redacted>
* chore: update webui build output

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Co-authored-by: Aleksander Grygier <redacted>