feat: console UX — restart button and session save/restore

[logosc]

Overview

Two UX improvements for the debug console that make iterative testing much faster:

1. Restart button

A Restart button in the header that kills the current agent session and starts a fresh one without reloading the page.

• Clears message history, tool log, usage counters, and error list
• Keeps the current user/model/API key/base URL settings
• Sends a POST /restart (or reuses /send with a special flag) to the backend, which cancels the running engine goroutine and resets Console state
• Button is disabled while no session is active (nothing to restart)
• Optional: confirm prompt if agent is mid-run ("Agent is running — restart anyway?")

Backend: Console needs a cancel context wired to the engine goroutine so it can be stopped cleanly. A new POST /restart handler cancels the context, waits for the goroutine to exit, then emits a clear SSE event so all connected frontends reset.

2. Save / restore session

A Save button (and matching Load) that snapshots the current conversation and lets the user restore it later.

Save

• Downloads a JSON file containing: messages, tool log, config (model, user, system prompt), usage totals
• File name: session-YYYY-MM-DD-HH-MM.json
• Pure frontend — no backend changes needed

Restore / Load

• A file-picker button that loads a previously saved JSON
• Replays the messages and tool log into the frontend state (no LLM calls — display only)
• Useful for sharing sessions or comparing runs

Implementation notes

• Backend restart: add runCtx context.Context + runCancel context.CancelFunc to Console[S]; handleSend uses runCtx for the engine goroutine; handleRestart calls runCancel(), waits for running to go false, then resets and emits clear
• Frontend save: JSON.stringify(state) → Blob → URL.createObjectURL → auto-click <a download>
• Frontend load: <input type="file"> → JSON.parse → dispatch synthetic actions to rebuild state

Open questions

• Should Load also send the session to the backend (so the agent sees conversation history on its next run)?
• Should the restart button optionally accept a new task prompt (like a "re-run with different input" flow)?