Add ESLint and Prettier

Prettier owns formatting: .prettierrc.json, format and format:check scripts, and
a one-off pass over the repo so the check is green from the start. Embedded
language formatting is off deliberately - the renderers build HTML in html
tagged templates and letting Prettier reflow those would change the markup
customers receive. docs/ is excluded because it is hand-wrapped prose, and shell
scripts have no Prettier parser.

ESLint covers the JavaScript: the two browser scripts, the build and check
scripts, and its own config. no-var is scoped out for the browser scripts on
purpose; they are plain ES5-shaped IIFEs and a hoisting difference would only
show up in a browser.

ESLint cannot lint the TypeScript here: typescript@7 is the native port with no
JavaScript compiler API, and typescript-eslint requires a 4.8-6.1 peer, so it
refuses to install and forcing it through would leave a parser that cannot read
the compiler. tsc --noEmit stays the TypeScript gate; README records the two
ways to change that.

lint and format:check now run in deploy.sh, and VS Code formats on save with
Prettier and surfaces ESLint findings.

package.json

@@ -16,6 +16,10 @@
     "verify": "node src/cli/verify.ts",
     "repair": "node src/cli/repair.ts",
     "typecheck": "tsc --noEmit",
+    "lint": "eslint .",
+    "lint:fix": "eslint . --fix",
+    "format": "prettier --write .",
+    "format:check": "prettier --check .",
     "test": "node --test test/*.test.ts",
     "check-dnd": "node scripts/check-dnd.mjs"
   },
@@ -28,7 +32,11 @@
     "sharp": "^0.35.4"
   },
   "devDependencies": {
+    "@eslint/js": "^10.0.1",
     "@types/node": "^26.6.2",
+    "eslint": "^10.11.0",
+    "globals": "^17.12.0",
+    "prettier": "^3.9.8",
     "typescript": "^7.0.2"
   }
 }