perf: use web workers for non-blocking builds

[daveades]

What

Refactors the compilation flow so that the API fetch and stream processing happen in a background Web Worker, keeping the main thread fully free for typing and UI interaction during long builds.

Files

ide/public/workers/compile.worker.js (new — Web Worker script)
Served as a static asset. Listens for compile messages, calls the configured compile endpoint via fetch(), and streams the response back chunk-by-chunk using postMessage. Supports cancellation through an AbortController keyed by job id; responds to cancel messages. Handles JSON and streaming responses identically to the old readCompileResponse logic.

ide/src/lib/compilationWorker.ts (new — Worker lifecycle manager)
CompilationWorker class: lazy spawn (never runs during SSR), typed message passing, cancellation forwarding, and automatic restart on crash (up to 3 attempts). All pending jobs are rejected immediately on crash so callers always get a definitive result.

ide/src/hooks/useCompilationWorker.ts (new — React hook)
Creates a CompilationWorker lazily on first use, tears it down on unmount. Returns compile(options) and cancel(). Applies formatTerminalChunk to each raw chunk before forwarding to the caller.

ide/src/features/ide/Index.tsx (modified)
handleCompile now delegates the fetch + stream to workerCompile(). Main thread only runs diagnostics parsing and Zustand state updates — both are lightweight. A new cancellation branch (error.cancelled) resets state to idle without logging a failure.

ide/src/components/ide/Toolbar.tsx (modified)
Added onCancelCompile prop. A Cancel button renders next to Build while isCompiling is true so users can abort a running build.

How it satisfies the acceptance criteria

Criterion	How
Compilation status tracked via message passing	Worker posts chunk, done, error, cancelled messages; main thread updates Zustand state accordingly
Main thread free during builds	fetch + stream loop runs entirely inside the worker; main thread only receives postMessage callbacks
Cancel a build in progress	cancel() sends an AbortController.abort() signal to the active fetch; toolbar exposes a Cancel button
Worker crash recovery	CompilationWorker.handleCrash rejects all pending jobs and auto-restarts up to 3 times

closes #439

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