feat(formatters): migrate all 14 Formatters onto the shared tool kit - #307

Phase 1 of the tool-consistency pass. Every Formatter now renders through
ToolShell + IOPanel/ToolTextArea/CodeEditor instead of hand-rolled Cards +
raw textareas, so they share one visual language (panel surfaces, headers,
borders, spacing, states) with base64/json-formatter.

Refactored (logic preserved — same state, handlers, i18n, editors):
- beautify-minify, json-schema-generator, json-to-code, json-diff,
  jsonpath-playground, yaml-formatter, sql-formatter, format-converter,
  markdown-preview-html, json-visualizer, diff-checker, graphql-formatter,
  regex-tester
- json-formatter kept as the gold reference (unchanged)

Notes:
- CodeEditor panes (sql/yaml/graphql/format-converter) kept as-is inside the
  new panels; json-visualizer's graph pane surface aligned to the kit while
  its canvas + floating node card are untouched.
- markdown-preview-html and graphql keep their tabs; regex-tester keeps its
  highlight-overlay textarea and match navigation.

0 type errors; all 14 pages render.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>