feat: concurrent chat panes and on-device SenseVoice input

[zephyr7zhang]

Summary

This PR adds two local-only features:

1. Concurrent conversations — switching sessions no longer unmounts the previous chat. Open conversations stay mounted in tabbed panes (up to 8), so an in-flight agent run keeps going in the background. Esc / abort only affects the visible pane.
2. On-device voice input — a microphone button next to the composer records in the browser and transcribes with local SenseVoice (sherpa-onnx-node). Audio never leaves the machine, and no cloud STT API keys are used or committed.

Also included:

• Do not pass empty toolNames: [] when creating a session (avoids merging/creating a tools-off session).
• New composers no longer inherit a stored "none" (chat-only) tool preset, so coding tools stay available by default.
• i18n strings for en / zh-CN / zh-TW.

Why

Today, selecting another session tears down ChatWindow / useAgentSession. That stops the running agent and makes it impossible to work on two chats at once.

Voice input is fully local (Hugging Face / hf-mirror model files under ~/.pi/voice/). Nothing in this PR reads ~/.pi/agent/auth.json, env API keys, or other credentials.

How it works

Panes

• lib/chat-panes.ts tracks open panes.
• AppShell keeps every open ChatWindow mounted and toggles isActive.
• Inactive panes keep their SSE/agent session alive, but do not steal the shared input ref, Esc handler, or top-bar stats.
• Idle panes are evicted first when the cap is hit; running panes are kept.

Voice

• POST /api/voice/prepare downloads the SenseVoice int8 model on first use.
• POST /api/voice/transcribe accepts WAV bytes and returns text.
• Right-click the mic button to cycle language: auto / zh / en.

Tests

• lib/chat-panes.test.mjs
• lib/wav-pcm.test.mjs
• components/AppShell.workspace-memory.test.mjs (mounted-pane assertion)
• lib/i18n/registry.test.mjs key parity

Out of scope (not in this PR)

Local Windows launchers and a machine-specific fs.readlink patch under scripts/ were left untracked on purpose.

中文摘要

• 多对话页签：切换会话时不再拆掉正在跑的 agent，后台对话可以继续跑。
• 本地语音输入：用 SenseVoice 在本机转写，不上传音频，也不需要/不会提交任何 API 密钥。