feat: Dynamic multi-terminal UI with split-pane support

[VivanRajath]

Summary

Adds a fully dynamic multi-terminal interface to the ClawLess terminal panel. Users can now open multiple independent shell sessions, rename them inline, close them individually, and view them in a split-pane layout alongside the Agent terminal — all without any page reload.

Motivation

The original UI had a single fixed terminal pane tied to the agent. Power users working with ClawLess often need a separate shell for running commands while the agent is active. This PR addresses that gap with a scalable, tab-based terminal system.

File Changes

File	What changed
src/container.ts	Replaced fixed shellProcess2/shellWriter2 fields with a Map<string, WebContainerProcess> and a generic spawnShell(id, terminal) method supporting unlimited concurrent shells
src/sdk.ts	Added spawnShell(id, terminal) passthrough; removed hardcoded _terminal2 and startShell2 calls
src/ui.ts	Rewrote bindTerminalTabs() — dynamic tab creation, inline rename (double-click), tab close (×), split-pane toggle, and drag-to-resize handle
index.html	Updated terminal panel markup: ⚙ Agent tab + ＋ button + ⊞ split button; cleaner pane structure
src/style.css	New styles for dynamic tabs, close button (hidden until hover), ＋ button, inline rename via contenteditable, scrollable overflow tab bar, split-pane layout

How to Test

Environment: Windows 11, Node.js v20, Chrome 123

1. npm install && npm run dev
2. Open http://localhost:5173 in Chrome
3. Wait for the boot sequence to complete (agent installs)

Tab creation:

• Click ＋ → "Terminal 1" tab appears; a live jsh shell launches inside the WebContainer
• Type ls → lists workspace files independently from the agent
• Click ＋ again → "Terminal 2" opens with another independent shell
• Both terminals run simultaneously without interfering with each other

Rename:

• Double-click "Terminal 1" tab label → it becomes editable (cursor appears)
• Type "Server" → press Enter → label saves as "Server"
• Click away also saves the rename

Close:

• Hover any dynamic tab → × button appears
• Click × → tab and its shell pane are removed; view switches back to Agent

Split view:

• Click ⊞ → terminal panel splits: active shell on right, Agent on left
• Drag the divider between panes to resize
• Click ⊞ again → returns to single-pane view

No regressions:

• Agent / gitclaw terminal works normally throughout
• npm run build passes with zero TypeScript errors
• No console errors observed

Checklist

• npm run build passes
• No TypeScript errors (strict mode enabled)
• Manually tested on Windows 11 / Chrome
• Backward compatible — existing startShell / startGitclaw flow untouched
• No new dependencies added