feat(ui): scaffold packages/ui with theme foundations

[EhabY]

🤖 This PR was created by Coder Agents on behalf of @EhabY.

Closes DEVEX-624.

Scaffolds the shared component library for the Workspaces webview work. Foundations only — no components yet (DEVEX-618/619/620 build on this).

• packages/ui — React + TS package wired like existing packages (catalog deps, root Storybook/ESLint/typecheck globs pick it up automatically). Consumed as source like webview-shared, but shaped for a future standalone NPM split: proper exports map (., ./tokens.css, ./codicon.css), react as the only peer dependency, @vscode/codicons (asset-only) as the only runtime dependency, no workspace:* runtime deps. A short README documents the theming model and the rules that keep the package splittable.
• tokens.css — semantic --ui-* tokens mapped to --vscode-* variables; the single place the library is theme-matched to VS Code. Same pattern as Microsoft's Webview UI Toolkit design-token layer, but pure CSS: components style against a small curated semantic set instead of scattering raw --vscode-* references (VS Code exposes 500+). High contrast needs no JS edge-case handling — --vscode-contrast* vars only exist in HC themes, so --ui-contrast-* tokens use transparent fallbacks and become no-ops elsewhere.
• useVscodeTheme — the issue asked to move it from webview-shared, but no such hook existed anywhere in the repo, so it is implemented here instead: returns the active theme kind from the data-vscode-theme-kind body attribute VS Code maintains in webviews, synced via MutationObserver + useSyncExternalStore. No re-export from webview-shared since there are no consumers yet.
• Storybook — trivial UI/Foundations story rendering the token swatches and current theme kind. The theme decorator now also mirrors data-vscode-theme-kind on the body so theme-aware hooks behave in Storybook as they do in real webviews.

Testing

• test/webview/ui/useVscodeTheme.test.ts — theme-kind mapping, missing-attribute default, re-render on theme change.
• pnpm typecheck, pnpm lint, pnpm format:check, pnpm test, pnpm build, pnpm storybook:ci all pass.

Implementation plan & decisions

Context

• Monorepo already has packages/{shared,webview-shared,tasks,workspaces,netcheck,speedtest,mocks,storybook-utils} with pnpm catalog: deps (catalogMode: strict).
• Root .storybook globs packages/*/src/**/*.stories.@(ts|tsx) — a packages/ui story is picked up with zero config.
• Root typecheck:packages / build:webviews run pnpm -r --filter ./packages/* — packages without a build script are skipped (e.g. webview-shared, consumed as source via its exports map).

Finding: useVscodeTheme did not exist

The issue says "move useVscodeTheme from packages/webview-shared", but no such hook exists on main or any branch. Decision: implement it fresh in packages/ui (DOM data-vscode-theme-kind + MutationObserver via useSyncExternalStore).

Decisions

1. useVscodeTheme: implement fresh (nothing to move).
2. Build: source-consumed like webview-shared (no vite build), but kept easy to separate later — self-contained package, proper exports map, react peer dep, no workspace:* deps. A real build step only becomes necessary at the actual NPM split.
3. Codicon export: added now (./codicon.css re-exporting @vscode/codicons) since follow-up issues will use it.
4. webview-shared re-export: skipped — no consumers exist yet, and it would add a workspace dep.
5. Semantic token layer (vs raw --vscode-* per component, as vscode-elements does): the deprecated @vscode/webview-ui-toolkit validated the token-layer pattern but implemented it with a JS design-token system; a pure-CSS layer gets the same single theming point without the machinery, and gives the future standalone package one place to define non-webview fallbacks.

Deliverables

• packages/ui/{package.json,tsconfig.json,storybook.preview.ts,README.md} + src/{index.ts,tokens.css,codicon.css,useVscodeTheme.ts,foundations.stories.tsx,css.d.ts}
• @repo/ui aliases in vitest.config.ts (webview project) and test/tsconfig.json
• Storybook theme decorator mirrors data-vscode-theme-kind
• Unit tests in test/webview/ui/

[linear-code]

DEVEX-624