docs: add installation, privacy, changelog, and 0.1.0 release notes

Phase 6 release collateral:
- README: Installation (unsigned-build/SmartScreen note, requirements,
  first-run downloads) and Privacy & local-first sections
- CHANGELOG.md seeded with the 0.1.0 feature set
- docs/release-notes-0.1.0.md draft for the GitHub Release body

Also corrects the CLIP model download size from ~330 MB to ~580 MB (the
actual model.safetensors is 577 MB) across the docs and the onboarding UI.

src/components/onboarding/StepAiFeatures.tsx

@@ -98,7 +98,7 @@ export function StepAiFeatures() {
           <p className="mt-1 text-xs leading-relaxed text-gray-500">
             Powers <code className="rounded bg-white/[0.07] px-1 py-0.5 text-[11px] text-gray-200">/s</code> search,
             "find similar", and the Explore view, so it's part of the standard pipeline: the CLIP model
-            (~330 MB) downloads automatically the first time embeddings run. Nothing to do — you'll see it
+            (~580 MB) downloads automatically the first time embeddings run. Nothing to do — you'll see it
             in the background-tasks bar.
           </p>
         </div>