Remove 278-line dead ScreenshotService and fix fragile FormData Content-Type handling

[Copilot]

ScreenshotService was never imported anywhere — screenshot capture lives in the service worker and watermarking in offscreen.ts. buildAuthHeaders always injected Content-Type: application/json, relying on a post-hoc delete in request() to fix it up for FormData uploads — a silent breakage waiting to happen on any pipeline refactor.

Changes

• Deleted src/services/ScreenshotService.ts — 278 lines of dead code, recoverable from git history if ever needed.
• ApiClient.buildAuthHeaders — accepts a new options?: { skipContentType?: boolean } parameter; skips setting Content-Type when the caller opts out rather than setting-then-deleting.
• ApiClient.requestWithAuth — passes skipContentType: true automatically when config.body instanceof FormData, making FormData upload behaviour explicit at the call site.
• README.md — removed stale ScreenshotService.ts entry from the key files list.

// Before: header set unconditionally, deleted later in request()
private buildAuthHeaders(customHeaders?: Record<string, string>): Record<string, string> {
  return { 'Content-Type': 'application/json', ...customHeaders, ... };
}

// After: caller controls whether Content-Type is included
private buildAuthHeaders(
  customHeaders?: Record<string, string>,
  options?: { skipContentType?: boolean }
): Record<string, string> { ... }

// requestWithAuth now signals intent explicitly
headers: this.buildAuthHeaders(config.headers, { skipContentType: config.body instanceof FormData })