feat(desktop): add polished cmux theme

[namredips]

Summary

• add a built-in cmux desktop theme with black chat surface, terminal-green prose, brighter green emphasis, lavender inline code, dark olive sidebars, and colored statusbar tones
• add fallback-safe optional theme tokens for emphasis text, sidebar nav/group/session colors, backdrop opacity, and statusbar item tones
• make Shiki code highlighting theme-aware so cmux fenced code blocks use an everforest-dark palette instead of the default blue-heavy theme

Validation

• cd apps/desktop && npx tsc -p . --noEmit
• cd apps/desktop && npx eslint src/components/chat/shiki-highlighter.tsx src/themes/context.tsx src/themes/presets.ts src/themes/types.ts src/app/chat/sidebar/index.tsx src/app/shell/sidebar-label.tsx src/components/assistant-ui/markdown-text.tsx src/app/chat/sidebar/session-row.tsx src/app/shell/hooks/use-statusbar-items.tsx src/app/shell/statusbar-controls.tsx src/components/Backdrop.tsx (passes; pre-existing warning at src/app/chat/sidebar/index.tsx:536)
• cd apps/desktop && npx vitest run --environment jsdom src/themes/
• hermes desktop --build-only --force-build
• extracted packed app.asar and verified cmux/token markers are present

Stack note: this is intentionally separate from #49902 (text size + chat width). This PR is only theme/chrome polish.

[teknium1]

Thanks for the focused desktop-theme contribution. The cmux preset is still absent from current main (apps/desktop/src/themes/presets.ts:280-289), so the feature remains relevant.

Problems

• sidebarWorkspaceForeground is defined and assigned by this PR, but workspace labels still inherit --ui-text-secondary: the shared rowLabel is defined in apps/desktop/src/app/chat/sidebar/chrome.tsx:31, and apps/desktop/src/app/chat/sidebar/projects/overview-row.tsx:130-136 does not override it. The requested blue workspace/profile names will therefore not render.
• The new optional-token cleanup in apps/desktop/src/themes/context.tsx has no regression coverage. apps/desktop/src/themes/profile-theme.test.ts verifies preference persistence, not CSS-variable cleanup after a theme switch.

Suggested changes

• Route workspace/profile row labels through --ui-sidebar-workspace-foreground, retaining the existing secondary-text fallback.
• Add a jsdom test that verifies optional theme variables are removed when switching from cmux to a theme that omits them.

The PR is currently marked dirty and postdates a desktop TypeScript conversion (39d09453f), so salvaging it will require conflict-aware integration rather than a clean cherry-pick.

Automated hermes-sweeper review.

[teknium1]

sidebarWorkspaceForeground is not consumed by the workspace-row path: SidebarRowLink inherits rowLabel from sidebar/chrome.tsx, which remains text-(--ui-text-secondary). Please wire this token into that consumer (with the existing value as fallback), otherwise this cmux color has no visible effect.