The board's panel grid is part of §5's board component: .bgrid and .bp-* move from content.ts into it #4833

Intent: the grid that lays out a board's panels, used by the board page and the studio's pinned panels, is named as part of §5's board, not as page furniture.

Done:

• [ ] the .bgrid, .bp-w2, .bp-w3, .bp-h2, .bp-h3 rules (and .bgrid's media rule) move unchanged from src/web/style/content.ts to src/web/style/components.ts
• [ ] docs/DESIGN.md §5's board row names .bgrid and the bp-w*/bp-h* size classes as its parts, and they leave the §5d content.ts list
• [ ] src/web/pages/boards-view.ts and src/web/studio/pinned.ts are not changed; a test asserts each class is defined once in the served stylesheet

Files: src/web/style/content.ts, src/web/style/components.ts, docs/DESIGN.md and one test. Do not touch .bform or .bmanage: other issues own them.

Run bun scripts/design-lint.ts and bun test test/design-lint.test.ts before pushing; lower docs/design/baseline.json only where a count fell, never raise it. Keep the diff to the files named.