Fix: Update Instagram detection logic

[Sumit0x00]

This PR fixes the broken Instagram detection which was causing false negatives (existing accounts reported as "Not Found").

Problem

1. Proxy Blocked: The previous implementation relied on imginn.com, which is currently returning a 403 (Cloudflare challenge) for automated requests.

2. Soft 404s: Direct probes to instagram.com return a 200 OK status for both existing and non-existing accounts.

3. UA Sensitivity: Standard desktop User-Agents often lead to a generic login wall, making it impossible to differentiate account existence via HTTP headers.

Solution

1. Switched to Message Detection: Changed errorType from status_code to message.

2. Mobile User-Agent: Updated headers to use a Mobile (iPhone) User-Agent. My testing confirmed that Instagram serves unique <title> tags for existing profiles when queried with a mobile UA, even without a session.

3. Error Patterns: Added detection patterns for non-existent profiles:

• <title>Instagram</title> (The generic title returned for the "unavailable" page).
• Profile isn't available.

4. Clean Up: Removed the non-functional urlProbe (imginn.com).

Verification Results
Tested locally with the --local flag:

• Existing Account: instagram.com/blamesumit -> Found
• Non-existent Account: instagram.com/random_handle_123 -> Not Found

[github-actions]

Automatic validation of changes

Target	F+ Check	F- Check
Instagram	❌   Fail	✔️   Pass

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Failures were detected on at least one updated target. Commits containing accuracy failures will often not be merged (unless a rationale is provided, such as false negatives due to regional differences).