perf: faster plugin updates, half-size first bundle, images from the chat attach on Windows

- Chat view: an attached image's path is pasted on its own after the text
  (promptForPane). claude turns a paste that is only an image path into
  [Image #n], but left a Windows path inside a paste of text as a plain path.
  Other files and @-mentions stay in the text as before.
- Plugin bootstrap: depsHash covers only what npm installs (no release
  version), so a version bump no longer reinstalls runtime deps; the client's
  installed tree is kept while its manifests are unchanged (no npm ci per
  update); the user-side build runs vite only (tsc -b cost ~25 s / ~900 MB);
  clientBuild is recorded only after a build that did not fail, so a failed
  one is retried; npm output goes to runtime/npm-install.log.
- Server: a port held by another program ends with a clear message and exit 1
  instead of an uncaught 'error' stack trace.
- Client: the onboarding wizard is lazy and rendered only before onboarding;
  usePrefersReducedMotion moved to hooks/ so PetSettings no longer pulls the
  Sagi chat/pane stack into the entry chunk (884 KB -> 431 KB, gzip 268 -> 133 KB).

client/src/components/Sagi/Sagi.tsx

@@ -78,6 +78,7 @@ import {
 } from "react";
 import { useNavigate } from "react-router-dom";
 import { useSettingsHref } from "../../lib/settingsLink";
+import { usePrefersReducedMotion } from "../../hooks/usePrefersReducedMotion";
 import { useTranslation } from "react-i18next";
 import { SagiAvatar } from "./SagiAvatar";
 import { PetSprite } from "./PetSprite";
@@ -126,23 +127,6 @@ interface Anchor {
   dragging: boolean;
 }
 
-/** The OS "reduce motion" preference, followed live. */
-export function usePrefersReducedMotion(): boolean {
-  const [reduced, setReduced] = useState(
-    () =>
-      typeof window !== "undefined" &&
-      window.matchMedia?.("(prefers-reduced-motion: reduce)").matches
-  );
-  useEffect(() => {
-    const mq = window.matchMedia?.("(prefers-reduced-motion: reduce)");
-    if (!mq) return;
-    const onChange = () => setReduced(mq.matches);
-    mq.addEventListener?.("change", onChange);
-    return () => mq.removeEventListener?.("change", onChange);
-  }, []);
-  return reduced;
-}
-
 /**
  * Fixed-position wrapper that places its content next to the avatar and clamps
  * it inside the viewport. It measures itself (and re-measures on content/size