fix: improve screenshot reliability with page readiness check

[RapierCraft]

Summary

• Improves comet_screenshot reliability by waiting for page to be ready before capturing
• Adds proper error handling and validation for empty screenshot data

Problem

Users reported that comet_screenshot returns empty content without any error message (#2).

Root Cause Analysis

The original implementation had several issues:

1. No connection resilience (withAutoReconnect wrapper missing)
2. No page readiness check - screenshot could be captured before content rendered
3. No validation of returned data - empty results passed silently
4. No error context when failures occurred

Solution

async screenshot(format: "png" | "jpeg" = "png"): Promise<ScreenshotResult> {
  return this.withAutoReconnect(async () => {
    this.ensureConnected();

    // Wait for page to be ready before capturing
    try {
      await this.client!.Runtime.evaluate({
        expression: `
          new Promise((resolve) => {
            if (document.readyState === 'complete') {
              resolve(true);
            } else {
              window.addEventListener('load', () => resolve(true), { once: true });
              setTimeout(() => resolve(true), 3000); // Fallback timeout
            }
          })
        `,
        awaitPromise: true,
        timeout: 5000,
      });
    } catch {
      // Continue anyway - page might still be usable
    }

    // Small delay to ensure rendering is complete
    await new Promise(resolve => setTimeout(resolve, 100));

    const result = await this.client!.Page.captureScreenshot({
      format,
      captureBeyondViewport: false,
    });

    if (!result || !result.data) {
      throw new Error(
        "Screenshot returned empty data. Ensure you're connected to a visible tab with content."
      );
    }

    return { data: result.data };
  });
}

Changes

• Added withAutoReconnect wrapper for connection resilience
• Added page readiness check (waits for document.readyState === 'complete')
• Added 100ms delay after load to ensure rendering completes
• Added captureBeyondViewport: false for consistent behavior
• Added validation with descriptive error message if data is empty

Test Plan

• Test screenshot on Perplexity main tab
• Test screenshot immediately after navigation
• Test screenshot when connection drops mid-operation
• Verify error message appears when screenshot fails

Fixes #2