Allow viewing conversations even when llama server is down (#16255)

Allow viewing conversations even when llama server is down (#16255)

* webui: allow viewing conversations and sending messages even if llama-server is down

- Cached llama.cpp server properties in browser localStorage on startup, persisting successful fetches and reloading them when refresh attempts fail so the chat UI continues to render while the backend is unavailable.
- Cleared the stored server properties when resetting the store to prevent stale capability data after cache-backed operation.
- Kept the original error-splash behavior when no cached props exist so fresh installs still surface a clear failure state instead of rendering stale data.

* feat: Add UI for `props` endpoint unavailable + cleanup logic

* webui: extend cached props fallback to offline errors

Treat connection failures (refused, DNS, timeout, fetch) the same way as
server 5xx so the warning banner shows up when cache is available, instead
of falling back to a full error screen.

* webui: Left the chat form enabled when a server warning is present so operators can keep sending messages

e.g., to restart the backend over llama-swap, even while cached /props data is in use

* chore: update webui build output

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Co-authored-by: Pascal <redacted>