chore: add prettier + enforce LF line endings

- Add .prettierrc (double quotes, 2-space, trailing comma es5, LF, 100 cols)
- Add .prettierignore (excludes node_modules, .next, server/, lock files)
- Add .editorconfig (LF + per-language indent rules for all editors)
- Expand .gitattributes to cover all text file types with eol=lf
- Add prettier@^3.5.3 devDep at workspace root with format/format:check scripts
- Add format/format:check scripts to web/package.json

.prettierignore

@@ -0,0 +1,18 @@
+# Dependencies
+node_modules/
+web/node_modules/
+
+# Build outputs
+web/.next/
+web/tsconfig.tsbuildinfo
+web/next-env.d.ts
+
+# Python / server
+server/
+
+# Lock files
+pnpm-lock.yaml
+web/pnpm-lock.yaml
+
+# Generated
+web/public/