settings: move the theme/motion panel into @hive/shared, add motion to agent

Review feedback on this PR (mara): "i think the component should be
shared. motion setting is missing." Both addressed:

- `settings-storage.ts` (generic localStorage hook), `theme-apply.ts`,
  `motion-apply.ts`, and `SettingsMenu.tsx`/`.css` all move from
  swarm-ui's `lib/`/`shell/` into `@hive/shared/src/settings/` —
  agent's previous local copies are deleted outright rather than kept
  as a second implementation. One component, `Badge` trigger
  everywhere (already used elsewhere in swarm-ui, so not a new visual
  language there either) — storage keys stay caller-owned (`themeKey`/
  `motionKey` props + matching `useApplyThemeOverride`/
  `useApplyMotionOverride` calls at each package's single mount point)
  so agent and swarm-ui keep fully independent, non-colliding
  persisted settings.
- Agent's settings menu now includes the motion row, matching
  swarm-ui's. No animation in the agent package is gated behind
  `data-motion` yet — same as when swarm-ui first built this plumbing
  ahead of having a consumer — so it's currently inert there, ready for
  whenever agent grows a motion-guarded animation.
- swarm-ui's own theme default flips to `'dark'` as part of this move
  (`theme-apply.ts`'s new default), superseding PR #3715 — that PR
  becomes redundant once this lands and will be closed rather than
  merged, to avoid the two colliding on the same file.

Verified end-to-end with real screenshots on both pages: shared
component renders identically (Badge trigger, theme+motion rows, dark
default) on agent's mock server and a static rebuild of swarm-ui's
dist.

frontend/packages/swarm-ui/src/shell/Shell.css

@@ -12,8 +12,8 @@
 
    Page-switch animation (see Shell.tsx's file-top comment for the JS
    half): both the nav indicator's slide and the page-body pop follow
-   the same three-rule motion-guard shape `lib/motion-apply.ts`'s own
-   doc comment specifies — a base rule,
+   the same three-rule motion-guard shape `@hive/shared/motion-apply.js`'s
+   own doc comment specifies — a base rule,
    `@media (prefers-reduced-motion: reduce)` to respect the OS default,
    `:root[data-motion='reduce']` as the explicit override (same
    specificity as the media rule, so source order after it wins), then