docs: update social preview tagline and assets

[ericodx]

Summary

Update the GitHub social preview SVG and PNG with a new tagline: "Detect and eliminate duplicated logic in Swift and Objective-C/C codebases to improve maintainability and code quality." Bullet items and separator repositioned to accommodate the extra line.

Type of Change

• docs: Changes only to the documentation.

Invariants Checklist

• Detection is deterministic (same input → same output)
• Source files are never modified (tool is read-only)
• Source locations are accurate (file, line, column)
• No false negatives introduced (previously detected clones are still detected)
• Performance not degraded for large codebases
• Swift 6 Strict Concurrency compatible
• Pipeline stages remain stateless pure transformations

Pipeline Impact

Which stages are affected?

• FileDiscovery
• Tokenization
• Normalization
• Detection (Type 1 / Type 2 / Type 3 / Type 4)
• Reporting
• CLI / Configuration
• Cache / Baseline
• None

Testing

• Unit tests added or updated
• Tests use TokenFactory / TempFileHelper / AnalysisHelper where appropriate
• Integration tests added or updated (if detection or pipeline logic changed)
• Both positive cases (expected clones) and negative cases (false positives to avoid) covered
• All tests pass locally (swift test)