feat(tasks-ui): migrate the Tasks page to shadcn/ui (Tailwind v4 + radix, lazy chunk)

Visual base replacement for the Tasks page after the 'hard shadows make a
dense graph unreadable' review. Behaviour, hooks, aria labels and i18n are
unchanged; the base layer is not.

- FOUNDATION: Tailwind v4 wired into tsdown's css-inline plugin (theme +
  utilities ONLY — preflight is excluded because the plugin's stylesheet is a
  global <style data-plugin> tag and would repaint the host page), shadcn
  tokens bridged to --dsw-* so the DSH theme keeps flipping light/dark with no
  dark: overrides, cn() util, components.json for the CLI, src/client/ui/.
- COMPONENTS: shadcn's component source vendored into src/client/ui (button,
  card, badge, separator, input, textarea, tooltip, popover, dropdown-menu,
  scroll-area, collapsible, toggle-group, skeleton, spinner, empty) with a
  documented local diff: lucide-react replaced by host IconXxx (no new icon
  dep), shadow-* dropped from non-floating variants, unused variants pruned,
  and React.forwardRef restored on Button — the host runs React 18, where
  radix's Slot cannot hand a ref to a plain function component (dev warns,
  production silently mis-anchors the floating layer).
- SHADOWS: static panels are now hairline + surface tiers with hover:bg-muted;
  shadows exist only on floating layers. Settled nodes recede by ink
  (text-muted-foreground/foreground-3) instead of opacity.
- DENSITY: node cards widened 132 -> 150px with a two-line clamped title
  (13px text in a 132px card left ~7 CJK characters), reserved height 46 -> 60.
- CHUNK: the whole page (graph, tree, task window, board, drawer, shadcn/radix
  layer and its stylesheet) moved into the lazy 'tasks' chunk. Core bundle
  988.5 -> 835.6 KiB (-153 KiB); the 709.8 KiB UI payload loads only when the
  Tasks tab opens.
- MODEL: workflow members whose childId is already a real node elsewhere are
  no longer synthesized into a duplicate id (React keys/edges), with a
  last-resort per-id dedupe.
- GUARDS: tests/ui-foundation.spec.ts, tests/ui-bundle.spec.ts (core/chunk
  split + size budget + no preflight/oklch/lucide), tests/ui-shadows.spec.ts,
  extended tests/theme.spec.ts (no colour literal / palette class in
  src/client/ui/** and the migrated files).
- REVIEW FIXES: portal popovers re-scoped under .dsw-tasks, graph loading
  state, 11px floor, error nodes no longer tint a whole border, TeamBoard's
  delete item is neutral (the window owns the two-step confirm).

Verified: typecheck / lint / 136 files 1502 tests / build bundle assertions /
mount e2e 7-7; browser harness 65-65 checks with 0 console errors.

scripts/ui-css.mjs

@@ -0,0 +1,32 @@
+#!/usr/bin/env node
+/**
+ * Compile the plugin's Tailwind entry (src/client/ui/theme.css) to a standalone
+ * stylesheet.
+ *
+ * The production path is tsdown's css-inline plugin (tsdown.config.ts), which
+ * inlines the compiled CSS into lib/client.js as a `<style data-plugin>` tag.
+ * The local visual harness (/tmp/dsh-visual) instead renders the REAL task-page
+ * components against a tokens stylesheet in a plain HTML page, so it needs the
+ * same utilities as a file it can `<link>` — this script is that file, built
+ * from the same pipeline (tsdown.config.ts exports `compileTailwind`) so the two
+ * cannot drift.
+ *
+ * Usage: node scripts/ui-css.mjs [outFile]   (default /tmp/dsh-visual/tailwind.css)
+ */
+import { mkdir, readFile, writeFile } from 'node:fs/promises'
+import { dirname, resolve } from 'node:path'
+import { fileURLToPath } from 'node:url'
+import { compileTailwind } from '../tsdown.config.ts'
+
+const ROOT = fileURLToPath(new URL('..', import.meta.url))
+const ENTRY = resolve(ROOT, 'src/client/ui/theme.css')
+const DEFAULT_OUT = '/tmp/dsh-visual/tailwind.css'
+
+const outFile = resolve(process.argv[2] ?? DEFAULT_OUT)
+// `from`/`base` stay anchored to the real entry file, so `@source` and
+// `@import` resolve against src/client/ui regardless of the cwd we run from.
+const source = await compileTailwind(await readFile(ENTRY, 'utf8'), ENTRY, { minify: false })
+
+await mkdir(dirname(outFile), { recursive: true })
+await writeFile(outFile, source)
+process.stdout.write(`ui:css -> ${outFile} (${source.length} bytes)\n`)