chore: switch code formatter from Prettier to Biome (#4223)

Replace Prettier with Biome for formatting JS/TS/JSX/CSS/JSON. The CI
format check drops from ~23s to ~0.4s.

- Unify config into a single root biome.json (formatter + linter); the
  former apps/readest-app/biome.json was linter-only
- Mirror the old .prettierrc.json style: 100 line width, 2-space indent,
  LF, single quotes, trailing commas
- Enable the CSS tailwindDirectives parser for @apply in globals.css
- Convert // prettier-ignore comments to // biome-ignore format:
- Root scripts and lint-staged now run biome; apps/readest-app lint runs
  `biome lint` (lint-only) so formatting stays a separate CI step
- Drop prettier + prettier-plugin-tailwindcss dependencies

Markdown/YAML are no longer format-checked (Biome does not format them)
and Tailwind class sorting is no longer enforced.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -12,8 +12,8 @@
     "prepare": "husky",
     "fmt:check": "pnpm --filter @readest/readest-app fmt:check",
     "clippy:check": "pnpm --filter @readest/readest-app clippy:check",
-    "format": "prettier --write \"**/*.{js,jsx,ts,tsx,css,json,md,html,yml}\"",
-    "format:check": "prettier --check \"**/*.{js,jsx,ts,tsx,css,json,md,html,yml}\""
+    "format": "biome format --write .",
+    "format:check": "biome format ."
   },
   "packageManager": "pnpm@11.1.1",
   "devDependencies": {
@@ -21,13 +21,11 @@
     "@sindresorhus/tsconfig": "^6.0.0",
     "husky": "^9.1.6",
     "lint-staged": "^16.2.7",
-    "prettier": "^3.3.3",
-    "prettier-plugin-tailwindcss": "^0.6.8",
     "typescript": "^5"
   },
   "lint-staged": {
-    "**/*.{js,jsx,ts,tsx,css,json,md,html,yml}": [
-      "prettier --write"
+    "**/*.{js,jsx,ts,tsx,css,json}": [
+      "biome format --write --no-errors-on-unmatched"
     ]
   }
 }