refactor: improve result pattern responses

[brunozoric]

Summary

• BrowserWindow abstraction — new DI feature (BrowserWindowFeature / NullBrowserWindowFeature) that decouples browser features from the global window object. LocalStorageCacheImpl now takes BrowserWindow as an injected dependency instead of checking typeof window directly.
• Result pattern for DirectoryTool — create() returns Result<void, DirectoryCreateError> instead of void. New createOrThrow() throws on failure. Internal callers (copy, copyOrThrow) and FileTool updated accordingly.
• Result pattern for FileTool — writeFile() returns Result<void, FileWriteError>, copy() returns Result<void, FileCopyError>. Cascaded into JsonFileTool.writeJson and PackageJsonFileTool.write which now propagate the Result.
• New typed errors — DirectoryCreateError, FileWriteError, FileCopyError with structured data payloads.

Test plan

• BrowserWindow — real impl exposes window.localStorage, null impl returns null, both via DI and factory functions
• LocalStorageCache — updated to register BrowserWindow dependency, all existing tests pass
• DirectoryTool.create — returns Result.ok() on success, Result.fail(DirectoryCreateError) on blocked permissions
• DirectoryTool.createOrThrow — throws DirectoryCreateError on failure
• DirectoryTool.copy/copyOrThrow — handles create Result internally
• FileTool.writeFile — returns Result.ok() on success, Result.fail(FileWriteError) when directory cannot be created
• FileTool.copy — returns Result.fail(FileCopyError) for missing source and blocked directories
• JsonFileTool.writeJson — propagates Result<void, FileWriteError>, failure test included
• PackageJsonFileTool.write — propagates Result<void, FileWriteError>, failure test included
• All 519 tests pass, coverage thresholds met

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