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.

README.md

@@ -36,6 +36,55 @@ A local-first desktop media library for browsing, filtering, and curating image
 | jpg, jpeg, png, gif, bmp | mp4, mov, m4v |
 | tiff, tif, webp, avif, heic, heif | webm |
 
+## Installation
+
+Phokus is a **Windows desktop app**. Download the latest installer from the
+[Releases page](https://github.com/JezzWTF/phokus/releases/latest) and run it.
+
+**Requirements:** Windows 10 or 11. The installer will fetch the WebView2
+runtime automatically if it isn't already present (it ships with Windows 11).
+
+### A note on the unsigned build
+
+Phokus 0.1.0 installers are **not code-signed**. Code-signing certificates are
+an ongoing expense that's hard to justify for an unfunded open-source project,
+so signing is on the roadmap rather than in place today.
+
+In practice this means Windows SmartScreen will show a blue
+**"Windows protected your PC"** warning the first time you run the installer.
+To proceed: click **More info → Run anyway**. If a release publishes a SHA-256
+checksum, you can verify the download against it first.
+
+### First run
+
+On first launch Phokus downloads a few tools and models — all one-time, and all
+processed on your machine:
+
+- **FFmpeg** (~tens of MB) for video thumbnails and metadata — downloaded in the
+  background; the guided first-run tour shows progress.
+- **WD tagger model** (~1.3 GB) — **optional**, only if you enable AI tagging.
+- **CLIP embedding model** (~580 MB) — downloaded automatically the first time
+  visual embeddings run (powers semantic search and similar images).
+
+## Privacy & local-first
+
+Phokus is local-first by design. **Your media and everything derived from it —
+thumbnails, embeddings, tags, ratings — never leave your machine.** There is no
+account, no telemetry, and no cloud sync.
+
+The only network activity is:
+
+- **One-time tool/model downloads** on first use (FFmpeg, the CLIP and WD
+  models, and the ONNX runtime), fetched from their official sources
+  (FFmpeg builds, Hugging Face, NuGet). These pull *tools to your machine* —
+  none of your images are uploaded.
+- **Update checks** against the GitHub Releases page, so the app can offer new
+  versions. This can be ignored if you never update.
+
+Everything Phokus stores lives in its app-data directory (`gallery.db`,
+`thumbnails/`, `models/`, `settings/`). Removing that directory resets the app
+completely; your original media folders are never modified.
+
 ## Stack
 
 - Tauri 2 + Rust backend