feat(dashboard): scaffold apps/dashboard with Vite + Solid + UnoCSS

[Exelord]

PR 7/10 of the dashboard track. Sets up the build pipeline and a
placeholder mount so subsequent PRs can land Solid components against
a working app shell.

Stack

• Vite 6 + vite-plugin-solid — dev server on :5280, proxies
  /api/* to 127.0.0.1:4280 (the legacy vzn dashboard server)
  so the new app can develop against real data immediately.
• UnoCSS — presetUno (Tailwind-compatible utilities),
  presetIcons (icons-on-demand via <i class="i-…">),
  transformerVariantGroup (hover:(text-fg bg-bg-muted) syntax).
• No web-fonts preset. @unocss/preset-web-fonts fetches from
  fonts.bunny.net at build time, which would break hermetic CI.
  System font stack only.
• Dark-by-default theme seeded with semantic color tokens (bg,
  border, fg, accent, ok/warn/err) so PR feat(dashboard): port Overview/Cache/Tasks/Runs pages to Solid #27's components
  can use stable class names regardless of palette tweaks.

Files

apps/dashboard/
  package.json        # @vzn/dashboard, private
  tsconfig.json       # Solid JSX preserve, types: bun + vite/client
  vite.config.ts      # plugins + dev proxy
  uno.config.ts       # presets + theme
  index.html          # mount point
  src/main.tsx        # render(<App/>, #root)
  src/App.tsx         # placeholder shell
  src/format.ts       # bytes / duration / relative-time helpers
  src/format.test.ts  # 3 describe blocks, 11 expectations

apps/dashboard/dist/ is covered by the root dist gitignore entry.

What this PR does NOT do

• Port pages — that's PR feat(dashboard): port Overview/Cache/Tasks/Runs pages to Solid #27 (Overview / Cache / Tasks / Runs +
  Run-detail with flamegraph).
• Touch the legacy vzn dashboard server or src/dashboard-ui/. The
  old static UI keeps serving as-is until PR feat(dashboard): serve apps/dashboard/dist; remove legacy dashboard-ui/ #28 wires the built
  Solid bundle in and removes the legacy files.

Test plan

• bun install resolves cleanly
• bun x vite build succeeds offline (no third-party fetches)
• bun run format clean
• bun run lint clean (oxlint + tsgolint)
• bun test packages/*/src/ apps/*/src/ — 268 pass, 0 fail

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