refactor: extract stateless JavaScript execution sandbox into internal package

[apzuk3]

Summary

This pull request radically restructures how Focusd executes custom user rules and classifications by decoupling the JavaScript engine from domain logic.

Key Changes:

• Stateless Sandbox Engine (@internal/sandbox): Extracted all v8go logic and esbuild transpilation out of the usage package into a generic, standalone execution module.
• Pluggable Contributors (Contributor Interface): Introduced a registry system where external packages (like usage or the new @focusd/core package) can globally inject polyfills, TS types, and native Go function callbacks (RegisterGlobals) directly into V8 isolates seamlessly on startup.
• Dynamic TypeScript Definitions via Wails IPC: Removed all hardcoded Monaco Type definitions from the React frontend. Instead, sandbox.GenerateTypes() writes out a combined types.d.ts file from all contributors on startup. Monaco then fetches these types dynamically using the new internal/fs/service.go securely through a Wails binding (ReadConfigFileSync).
• Cleaned Up Dependencies: Removed messy context injection (like passing MinutesUsedInPeriod closures per-execution). The usageContributor now queries the usage Service natively since it attaches the database query directly to the RegisterGlobals V8 isolate initialization.
• Removed iOS Build Artifacts: Cleaned up lingering broken Wails templates in build/ios.

This sets the stage for easy extensibility: if we want to add HTTP or filesystem capabilities to rules later, we only need to write a new Contributor and register it!