refactor: abstract platform dependencies (HttpContext, attachments)

[flojon]

Part of #60. Depends on #61.

What

Replace direct IHttpContextAccessor / HttpContext usage in shared code with platform-neutral abstractions. Two concrete implementations: one for web (reads from HttpContext), one for desktop (returns hardcoded local user).

Scope

• TenantContext — replace IHttpContextAccessor with a scoped ICurrentUser interface
• Attachment serving — wrap the /attachments/{id} HTTP endpoint behind an IAttachmentProvider so the desktop target can serve files directly without an HTTP request

Acceptance criteria

• ICurrentUser interface introduced; web implementation resolves from HttpContext, desktop implementation returns local user
• IAttachmentProvider introduced; web implementation proxies the existing HTTP endpoint, desktop implementation returns stream directly
• Web app builds and behaves identically to before
• No IHttpContextAccessor references remain in KoalaBooks.Components or shared service projects

Notes

Safe to merge independently. Improves the web app regardless of whether the desktop app ships.