revert(tasks-ui): drop the shadcn/Tailwind layer and return to the plugin's own UI system

shadcn's defaults are built for a standalone app: 14px body, 36px controls,
solid primary fills and elevation read as a foreign surface next to this
plugin's own tabs (files, changes, side chat, settings), which are 12/11/10px
type, 28px controls, hairline borders and flat panels. Retuning the layer did
not converge, so the layer goes.

- Removed: src/client/ui/** (21 vendored components), components.json,
  scripts/ui-css.mjs, the Tailwind/PostCSS compile step in tsdown, and the deps
  tailwindcss / @tailwindcss/postcss / postcss / radix-ui /
  class-variance-authority / tailwind-merge. (diff and simple-icons stay: they
  patch undeclared bare imports of the host primitives, unrelated to shadcn.)
- The seven page surfaces are back on the plugin's own base: *.module.css with
  --dsw-* tokens plus the host primitives (Button/Menu/Modal/Pill/Tag/Switch/
  Input/Tooltip/StateDot/MarkdownText). Behaviour, data-* hooks, aria labels,
  i18n keys and the AnchoredPopover geometry/drag/dismiss contract are
  unchanged; the non-styling fixes landed during the shadcn era (model id
  dedupe, task-window CAS, drawer arming, popover dragging, the chunk packaging
  glob) were re-applied and are covered by the existing specs.
- The tasks page returns to the core bundle: `tasks` left CHUNK_NAMES /
  CHUNKS / ChunkName, src/client/chunks/tasks.tsx is gone and tabs.tsx imports
  SubagentView directly.
- The reverted stylesheet was retuned to the plugin's measured vocabulary
  (12/11/10px, 28px controls, border-l2 with border-l1 hairlines, hover fills,
  no static shadows, --dsw-shadow-lv2/3 on floating layers only) and verified
  against a 70-assertion real-browser contract; the harness's own two
  measurement defects were fixed with a falsification test proving the
  assertions still catch injected regressions (56/70, 14 FAIL).
- Tests: ui-foundation / ui-bundle / ui-shadows (the shadcn guards) deleted,
  theme.spec scoped back to the plugin's own stylesheets, bundle-route and
  manifest-consistency chunk lists back to four. Docs: AGENTS.md and the plugin
  guide drop the Tailwind/shadcn guidance; the design doc marks those sections
  superseded and records why the layer was removed.

Verified: typecheck/lint clean; 133 files / 1410 tests; core bundle 987.3 KiB
with no radix-ui/--tw-/tailwind strings and no client-tasks.js; mount e2e 7/7;
20 real-browser screenshots at 360/720 for graph, tree, board, task window,
drawer (including armed), both popovers and the 77-node fixture.

src/client/chunks/tasks.tsx

@@ -1,20 +0,0 @@
-/**
- * Lazy chunk entry: the whole Tasks page (workflow graph + tree, task window,
- * team board, jobs drawer) together with the vendored shadcn/ui component set
- * it renders. Built as `lib/client-tasks.js` and registered under
- * `dsh-better-sidebar/tasks` — fetched only when the Tasks tab is first
- * opened (see chunk-loader.ts and docs/plans/2026-08-12-lazy-chunks-design.md).
- *
- * Why it is a chunk: the shadcn/radix/tailwind-ui layer costs ~430 KiB raw
- * (radix primitives + floating-ui + tailwind-merge + the vendored components),
- * and README's "startup pulls only the core bundle" contract must hold for
- * every user who never opens the Tasks tab. Never import this module (or the
- * page modules it re-exports) from the core bundle.
- */
-// The Tailwind sheet (theme bridge + compiled utilities) ships WITH the
-// chunk: it styles this page only, so the core bundle stays free of the
-// UI-layer payload and the host page never receives the stylesheet until a
-// reader opens the Tasks tab.
-import '../ui/theme.css'
-
-export { SubagentView } from '../SubagentView.tsx'