bug: message is silently lost when sending fails (e.g. network offline)

Problem

When sending a chat message fails (network offline, server unreachable, or a non-2xx response), the message silently disappears. The input is cleared, the optimistic user message flashes and is removed, no error is shown, and the text is gone. The only trace is an unhandled TypeError: Failed to fetch in the console.

Steps to reproduce

1. Open a chat and type a message (optionally attach a file).
2. Make the request fail, e.g. DevTools → Network → Offline, or stop the server.
3. Press Enter.

Actual: the message appears for a moment, then vanishes; the input and attachments are empty; no toast. For a new chat the tab title keeps the first 40 characters of the lost message.

Expected: an error is shown and the message (and attachments) stay in the input so it can be re-sent.

Root cause

In ChatPanel.svelte send():

• inputText = '' and chatInputEl.clearUploads() run before apiSendMessage is awaited.
• The catch block only removes the optimistic message and rethrows. Nothing restores the draft and no toast is shown.
• ChatInput calls onsend() without awaiting, so the rethrown error becomes an unhandled rejection and never reaches the user.
• The streaming/queue branch has the same problem (its catch only logs).

Proposed fix

• Capture the untransformed draft text and the attached files before sending.
• On failure, restore both, reset currentMessageId, and reset the new-chat tab label.
• Replace throw e with toast.error(...), matching how compact/fork failures are reported. ApiError messages from the server are shown as-is; other failures (network) use a new chat.sendFailed string.
• Apply the same restoration in the queue branch.
• ChatInput gets a small restoreUploads() counterpart to clearUploads().

Deliberately minimal: no retry button or "failed" marker on the message; happy to adjust if you prefer a different UX.

Verification

Playwright (headless Chromium) against a local build, aborting POST /api/chats with internetdisconnected:

Additional context

Implemented on a fork (4 files, +20/-3):
https://github.com/open-webui/computer/compare/main...ebifrier:computer:fix/keep-message-on-send-failure

Related but different cause: #262 (prompt deleted when moving the window) does not involve sending.

Start with send() in ChatPanel.svelte and the send callback plus upload helpers in ChatInput. Reproduce the failed POST with Playwright or an offline network, then verify against the issue’s checks: the text and attachments remain available, the failure is shown, the new-chat title resets, and no unhandled rejection occurs.