Add web app manifest for installability

Background

A web app manifest enables browser install prompts (Chrome desktop/mobile, Edge), theme integration with the OS, and is occasionally surfaced by Google as an "installable app" in search results.

Proposal

Add src/manifest.webmanifest and link it from every HTML page.

{
  "name": "HTML Tools",
  "short_name": "HTML Tools",
  "description": "Browser-native utilities for developers — no installs, no servers, no tracking.",
  "start_url": "/",
  "display": "standalone",
  "background_color": "#030712",
  "theme_color": "#030712",
  "icons": [
    { "src": "/icon-192.png", "sizes": "192x192", "type": "image/png" },
    { "src": "/icon-512.png", "sizes": "512x512", "type": "image/png" },
    { "src": "/icon-maskable.png", "sizes": "512x512", "type": "image/png", "purpose": "maskable" }
  ]
}

Tasks

• Create src/manifest.webmanifest
• Design src/icon.svg (simple "HT" or wrench mark on the brand background)
• In scripts/build.mjs, render icon-192.png, icon-512.png, and icon-maskable.png from the SVG via @resvg/resvg-js (already a devDep)
• Add <link rel="manifest" href="/manifest.webmanifest"> to all HTML files
• Add <link rel="apple-touch-icon" href="/icon-192.png"> for iOS install
• Update AGENTS.md to note the manifest convention

Notes

• The maskable icon needs ~10% safe-area padding from the icon edge.
• Coordinate with #_ (service worker) — manifest + SW together unlock the full installable PWA experience.

Start with the existing HTML files, scripts/build.mjs, and the current AGENTS.md conventions. Create src/manifest.webmanifest and src/icon.svg, then trace the build entry point to render the three PNG icons with the existing @resvg/resvg-js dependency. Done means every HTML page has both install links, the manifest references the generated icons, and AGENTS.md documents the convention.