fix(sheet): code block bg follows --card instead of theme-locked var

[winlp4ever]

Summary

Sheet code blocks were still leaking the old per-theme bg color through. PR #140 pinned `.shiki` (the inner highlighted layer) to `var(--card)`, but the parent wrapper `.code-block-body` was still painting a separate per-theme variable `--code-block-bg` (rose-pine #191724, tokyo-night #1a1b26, monokai #272822, etc.). When the Shiki `

` was shorter than the wrapper — empty block, short snippet, or the ~80ms pre-Shiki-load window — the leftover band at the bottom showed the theme-locked color instead of the card surface.

The fix
editor.css:265 — `.code-block-body` background now reads `var(--card)`. Parent + child share one source of truth, no more leaks.
`--code-block-bg` had exactly one reader (grep-verified before the change), so its 16 per-theme declarations across index.css are dead code after the rename — deleted in the same pass.
Diff shape
`+6 / -22` across 2 files:

editor.css — 1-line background swap + comment block updated to reflect the new contract ("wrapper matches --card so child + parent align")
index.css — 16 `--code-block-bg: #...;` lines deleted across the theme blocks (parchment, matcha, noir, catppuccin light/dark, tokyo-night, gruvbox light/dark, monokai-pro light/dark, rose-pine dawn/main, and the system default pair)

Test plan

 `npm run type-check` clean
 `npm test --run` — 320/320 green
 Empty Tiptap code block in a sheet (the reported case) — fully card-colored top to bottom, no stripe at the bottom
 Short code block (1-2 lines) — gap below Shiki layer is now card, not theme-locked
 Long code block — unchanged (Shiki fills the parent)
 Theme cycle across all 6 app themes × light/dark — code block bg tracks `--card` everywhere
 Streamdown code blocks in assistant messages — sanity check, expect no regression (Streamdown's own wrapper has no bg rule)