plugin-flow-builder: migrate eslint and prettier to biome

[Iru89]

Description

Migrates @botonic/plugin-flow-builder from ESLint + Prettier to Biome v2 as part of the ongoing monorepo linting migration (Phase 1, step 6/6).

• Replaced ESLint/Prettier scripts with Biome (lint, lint:check, format) and removed eslintConfig from package.json
• Formatted and auto-fixed 111 files across src/ and tests/ using biome check --write --unsafe
• Updated pre-commit hook to include botonic-plugin-flow-builder in the Biome pattern and removed the ESLint hook
• Updated .prettierignore and CI workflow (LINT_COMMAND: npm run lint:check)

Changes breakdown

Configuration files:

• packages/botonic-plugin-flow-builder/package.json — New lint, lint:check, format scripts using Biome; removed eslintConfig section
• .pre-commit-config.yaml — Added botonic-plugin-flow-builder to Biome hook pattern; removed dedicated ESLint hook
• .prettierignore — Added packages/botonic-plugin-flow-builder/
• .github/workflows/botonic-plugin-flow-builder-tests.yml — Added LINT_COMMAND: npm run lint:check

Source code (auto-fixed by Biome):

• Converted value imports to import type where only used as types (e.g., import { FlowContent } → import type { FlowContent })
• Removed unused imports (import React from 'react' in 3 files)
• Renamed unused variables with _ prefix (e.g., catch (error) → catch (_error))
• Wrapped single-line if statements with block statements
• Applied consistent formatting (single quotes, trailing commas, semicolons, indentation)

Test files:

• Removed unnecessary @ts-expect-error directives that became unused after import type conversions resolved the underlying type mismatches
• Applied same formatting fixes as source code

Remaining warnings (acceptable)

These are warnings only and do not block lint:check:

• noExplicitAny — Existing any types in the codebase (37 warnings)
• noUselessFragments — Intentional <></> empty returns in JSX (9 infos)
• noNonNullAssertion — Existing ! assertions

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