fix: remove metadata fields leading underscore

[dobromirts]

Closes #325

Frontmatter fields _description, _keywords, and _license were prefixed with underscores ,Astro's Zod schema strips unknown fields, so license: commercial never reached entry.data, leavingLicenseIndicator with no value and the heading badge invisible.

Changes

Frontmatter migration (221 files across angular + xplat)

• _license → license, _description → description, _keywords → keywords
• JP files: removed redundant EN originals below _language: ja

src/sidebar.ts

• "Overview" leaf nodes created from parent toc entries with href + items
  (or header: true) were hardcoded without badges — they now carry
  premium: true when the parent toc entry has "premium": true

toc.json (angular EN/JP, xplat EN/JP)

• Added "premium": true to overview/header entries whose child topics
  are already premium: Data Grid, Tree Grid, Hierarchical Grid, Pivot Grid,
  Spreadsheet, Charts, Geo Map, Excel Library, Transaction Service,
  Grids & Lists, Grids header (xplat)