fix: add PNG icon so the extension is publishable (#1)

package.json declares "icon": "icons/k9-icon.png" but only k9-icon.svg
existed. The VS Code Marketplace rejects SVG icons, so `vsce package`
would fail outright — a hard publish blocker.

Add a 128x128 RGBA PNG rendered faithfully from k9-icon.svg (same
diagonal #4ad97a->#1a6a2e gradient, 16px rounded corners, white "K9"
wordmark + faint accents), matching the format vscode-a2ml already
ships (128x128 RGBA). icons/k9-icon.svg is retained as the source.

This removes the only packaging blocker; publishing is now a single
`vsce publish` once the Azure DevOps PAT is available (see issue).

Refs #1

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: hyperpolymath