feat(editor): per-theme syntax tokens for CodeMirror and live preview

[jsgrrchg]

Summary

• Each of the 21 UI themes now drives its own syntax-highlight palette for CodeMirror (source mode) and for the static highlighter used in chat/diffs.
• The boundary is CSS variables (--code-*): applyThemeColors publishes them on :root, both buildSyntaxHighlightStyle() and .cm-static-token-* resolve through them, and theme changes propagate without any compartment reconfigure or themeName plumbing.
• Live preview headings and inline code now pick up --code-markup / --code-string so they match the source-mode coloring.
• Removes the unused @codemirror/theme-one-dark dependency.

Why

All 21 themes (Catppuccin, Gruvbox, Tokyo Night, Synthwave '84, etc.) previously shared oneDarkHighlightStyle (dark) or defaultHighlightStyle (light), so the code blocks and markdown markup looked identical across themes — defeating the point of having distinct themes for poweruser identity.

Approach

1. ThemeColors extends ThemeUiColors and adds codeAnchors: CodeColorAnchors (12 slots: comment, constant, escape, function, keyword, markup, parameter, property, string, type, typeParameter, variable).
2. applyThemeColors writes --code-* vars to :root next to the existing UI vars.
3. New extensions/syntaxTheme.ts defines a HighlightStyle mapping Lezer tags to var(--code-*). Coverage mirrors staticCodeHighlight.tsx so source mode and rendered code stay aligned.
4. getSyntaxExtension() no longer takes arguments; isDark plumbing and the reconfigure effects are removed.
5. Anchors for the 21 themes ported as-is from the curated palettes in a sibling project (canonical themes use their published colors; in-house themes are harmonized with the accent).

Test plan

• `tsc -b` clean.
• `eslint` clean on touched dirs.
• `vitest run`: 149 files, 1547 tests passing, 0 failing.
• Sub-agent audit + fixes (2 majors, 5 minors).
• Smoke manual: open a `.md` with headings/bold/italic/inline code/fences, cycle through `default`, `gruvbox`, `tokyoNight`, `synthwave84` × light/dark, validate source mode and live preview.
• Smoke manual: chat code blocks and agent diffs over add/remove/update backgrounds.

Out of scope

• Importing/exporting custom user themes.
• Porting accessibility contrast checks.
• Theming terminal, search panel, merge view, or diff semantic colors beyond what the static syntax highlighter already touches.