Add CI, fix taskbar icon, embed exe icon, 80% zoom

[vecnode]

Summary

• Adds .github/workflows/build.yml: builds Release distributables for Windows (MSVC) and Linux (GTK3/WebKit2GTK) on push/PR to main, uploaded as artifacts.
• Clarifies in README/AGENTS that the app needs no internet at runtime, except when opting into an online p5.js build from the Libraries panel (build-time dependency fetching is the only other internet use).
• Fixes icons/app_icon_small.ico: it was a solid black 16x16 square (crude 4bpp, no alpha) — regenerated as an anti-aliased black circle at multiple sizes.
• Adds icons/app.rc, wired into CMakeLists.txt, so app_icon.ico is embedded as neo-processing.exe's own PE resource icon (visible in Explorer/taskbar/Alt+Tab), independent of the runtime window-icon code in main.cpp. No extra step needed in build_and_distribute.bat.
• public/style.css: html { zoom: 0.8 } so the app starts more compact, giving the editor/preview more effective space in the same window.

Test plan

• Built locally via build_and_run.bat (Debug, MSVC) — succeeded.
• Verified icons/app_icon_small.ico renders as an anti-aliased circle at 16px (extracted and inspected the PNG-compressed frame).
• Verified the compiled neo-processing.exe's embedded PE icon shows the app logo (extracted via Icon.ExtractAssociatedIcon), confirming icons/app.rc is linked correctly.
• CI itself (Windows + Linux matrix) will be exercised on this PR — not verified locally for Linux (no Linux environment available here).
• Could not visually confirm the 80% zoom in the live webview window — the app's loopback-only HTTP server isn't reachable from the sandboxed browser pane by design (see AGENTS.md security notes), and desktop screenshot access wasn't available for this unlisted app.

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