chore: migrate dashboard from npm to pnpm

[pratyush618]

Summary

• Migrate the dashboard/ sub-project from npm to pnpm; pin pnpm@10.30.3 via packageManager in package.json
• Replace package-lock.json (3037 lines) with pnpm-lock.yaml (1817 lines, 110 packages)
• No runtime or user-facing impact — the dashboard bundle is pre-built and inlined into the Python wheel, so pip install taskito is unaffected

Changes

• dashboard/package.json — added "packageManager": "pnpm@10.30.3"
• dashboard/package-lock.json — removed
• dashboard/pnpm-lock.yaml — added
• .github/workflows/ci.yml — dashboard-lint job now uses pnpm/action-setup@v4, pnpm install --frozen-lockfile, pnpm exec …; setup-node uses cache: "pnpm" with cache-dependency-path: dashboard/pnpm-lock.yaml
• docs/guide/observability/dashboard.md — dev section uses pnpm install / pnpm run; added a corepack enable tip for contributors without pnpm

Test plan

• pnpm install clean install (~12s, 110 packages)
• pnpm run build → 139 kB dist/index.html
• pnpm run typecheck passes
• pnpm run lint (biome) passes, 46 files
• CI dashboard-lint job passes with --frozen-lockfile