fix(http): apply settings' httpIdleTimeoutMs to the Web UI

[QuickLyx]

Fixes #1126

Problem

registerNodeInstrumentation() called configureHttpDispatcher() with no timeout, so the Web UI always kept DEFAULT_HTTP_IDLE_TIMEOUT_MS (five minutes) and never read httpIdleTimeoutMs. That value is undici's bodyTimeout on the global dispatcher, an idle timeout that fires when the response body has had no data for that long, so a provider stream that stayed quiet for five minutes ended with a bare terminated while the same setting applied correctly to CLI sessions.

Change

• lib/http-dispatcher.ts — add readHttpIdleTimeoutMs(agentDir), which reads the agent directory's settings.json the way the CLI reads it. A missing file, a file that does not parse, an absent key, or a value the CLI would reject returns undefined, so the existing default stays in place. The read goes through readRegularFileText, so a FIFO or a device there throws instead of blocking startup.
• instrumentation-node.ts — pass the value through.
• lib/http-dispatcher.test.mjs — cover the reader: missing file, broken JSON, absent key, negative value, null, a non-object file, a configured value, 0, "disabled", a byte-order mark, and PI_CODING_AGENT_DIR.

Testing

node --experimental-strip-types --test lib/http-dispatcher.test.mjs
# tests 2 | pass 2 | fail 0

The existing proxy test still passes unchanged.

Notes

• Only the global settings file is read. The dispatcher is configured once at startup, before any project is selected, so a project's .pi/settings.json cannot apply here.
• The agent directory is resolved locally rather than through the SDK's getAgentDir(). This module runs in Next.js instrumentation, where importing the SDK costs about a second of startup time, while undici — the only other dependency here — costs about a tenth of one. The resolution mirrors getAgentDir(): PI_CODING_AGENT_DIR, ~ expanded, otherwise ~/.pi/agent.
• Without httpIdleTimeoutMs, behavior is unchanged: the five-minute default.