refactor(ui): migrate sidebar visual properties to cva variants

[Astro-Han]

Summary

Migrate sidebar SessionListPanel visual properties from hand-written .maka-* CSS to cva variants aligned with ui.tsx governance, and fix the collapsed Storybook story rendering.

Why

Closes #390. The sidebar carried ~180 lines of visual CSS (hover, active, focus, disabled, pending states for nav-row, settings, row-action) duplicating what cva variants already govern through buttonVariants in ui.tsx. Font sizes had drifted to seven tiers (9.5/10/10.5/11/12/13/14px) with no design intent. The collapsed story was blank because .agents-sidebar[data-collapsed="true"] { width: 0 } (correct in the app, where an outer resizable container sets width) collapsed the panel inside Storybook's 72px block container.

Scope

Changed:

• packages/ui/src/session-list-panel.tsx — add 3 cva variants (navRowVariants, settingsButtonVariants, rowActionVariants); unify all sidebar UiButton to size="nav" + cva variant.
• apps/desktop/src/renderer/styles/sidebar.css — remove ~180 lines of visual CSS; keep grid layout, -webkit-app-region, and stable class hooks only.
• apps/desktop/src/renderer/styles/onboarding.css — converge font sizes to 11/13/14px; remove uppercase/letter-spacing from group label; fix .maka-list-group-toggle font: inherit bug (was resetting weight/size to parent values); add justify-content: flex-start to override UiButton's justify-center.
• packages/ui/stories/session-list-panel.stories.tsx — collapsed story adds inline <style> overriding width: 0 so the 72px icon rail renders.
• apps/desktop/src/main/__tests__/session-row-actions-fail-soft-contract.test.ts — check cva pending state instead of CSS rule.
• apps/desktop/src/main/__tests__/search-modal-lifecycle-contract.test.ts — regex matches cn() className.

Not included:

• primitives/sidebar.tsx (740 lines, shadcn-style) remains dead code — out of scope.
• List-row container and list-row-main stay in CSS (single instance with state-coupled selectors, cva has no benefit).
• Action overlay gradient leak at 176px narrow width (low priority, visibility: hidden already mitigates meta/unread).

Verification

• npm run typecheck — all workspaces pass.
• npm run pretest (check-console + check-a11y) — pass.
• Sidebar contract tests (session-row-actions-fail-soft, search-modal-lifecycle) — 23/23 pass.
• Storybook visual check across all 6 stories: before/after screenshots in notes/screenshots/compare/.
• Collapsed story: 0 visible icons → 7 visible icons (search, toggle, 4 nav, settings).

User-facing impact

No behavioral change. Visual changes are within existing story contracts:

• Font sizes converge to 3 tiers (11/13/14px), reducing visual noise.
• Active-state hierarchy: nav active (0.09) > list active (0.07) > hover (0.06), clarifying which surface is primary.
• Group labels use weight-based hierarchy instead of uppercase/letter-spacing (better for Chinese).
• Radius unified to 8px (rounded-md); original 6px/8px mix had no design intent.

Reviewer notes

• Rebased on latest main after shell-chrome refactor moved search/toggle from sidebar header to shell topbar; sidebarIconButtonVariants was removed as it no longer has consumers in the sidebar.
• The collapsed story <style> override is a fixture fix, not a component change — in the real app the outer resizable grid column sets width, so width: 0 on .agents-sidebar[data-collapsed="true"] is correct.
• size="nav" is an empty-string variant (no size utilities) that prevents buttonVariants from injecting conflicting size classes; visual properties come entirely from the cva variant.
• Root font-size is 15px, so text-sm (0.875rem) = 13.125px, not the 14px a 16px root would give. list-row-name uses 0.875rem to match nav's text-sm.