feat(app): add the shadcn + tailwind design-system layer

[DeliciousBuding]

First step of the frontend rebuild: the shadcn/ui + Tailwind layer that the conversation UI will be built on. No component is migrated yet — this PR only makes the layer real, verified and mergeable, so the UI slices that follow are pure component work.

What it adds

• Tailwind v4 (tailwindcss + @tailwindcss/vite) plugged into both renderers (web, desktop), with the stylesheet imported once per entry.
• shared/src/styles/shadcn.css — the token bridge. The design system already exposed shadcn-shaped variables (--background, --foreground, --primary, --muted, --card, --border, --ring, --input, --popover, --accent, --destructive), so @theme inline maps Tailwind's colour namespace onto them and onto the existing --r-xs..xl radius scale. Palette and radius stay single-sourced in tokens-base.css / themes.css.
• Preflight is deliberately not imported. It resets margins, headings, lists and form controls globally and would silently restyle every component still on CSS Modules. Only the one preflight rule shadcn actually needs is re-declared (zero-width solid border so border utilities draw a real border). Full preflight lands with the last old-UI removal.
• shared/components.json so pnpm dlx shadcn@latest add … keeps working against this repo's aliases (@shared/shadcn/ui, @shared/lib/utils).
• shared/src/lib/utils.ts (cn() = clsx + tailwind-merge) and the vendored button component as the first official shadcn primitive, with a render test.

Components are vendored one at a time as a surface adopts them — the other eight primitives I pulled while wiring the registry were untested dead weight and are not in this PR.

Verified

• pnpm --filter agenthub-web build: passes; the emitted index-*.css contains --color-background / --radius-md, i.e. the theme bridge really is in the bundle.
• pnpm --filter @agenthub/shared typecheck, agenthub-desktop typecheck: pass.
• pnpm --filter @agenthub/shared lint: 0 errors (agenthub-web lint --max-warnings 0: pass).
• vitest run shared/src/shadcn: 2/2.

Next slices

1. Composer → shadcn InputGroup + Attachment + Button.
2. Transcript rows → Message / Bubble / Tool / Reasoning, keeping the existing transcript data model.
3. Sidebar + shell chrome, then delete the CSS Modules and the visual-QA baselines that covered them.

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