docs: move THIRD_PARTY.md to repo root

THIRD_PARTY.md

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+# Third-Party Notices
+
+FileRise bundles the following third‑party assets. Each item lists the project, version, typical on-disk location in this repo, and its license.
+
+If you believe any attribution is missing or incorrect, please open an issue.
+
+---
+
+## Fonts
+
+- **Roboto (wght 400/500)** — Google Fonts  
+  **License:** Apache License 2.0  
+  **Files:** `public/css/vendor/roboto.css`, `public/fonts/roboto/*.woff2`
+
+- **Material Icons (ligature font)** — Google Fonts  
+  **License:** Apache License 2.0  
+  **Files:** `public/css/vendor/material-icons.css`, `public/fonts/material-icons/*.woff2`
+
+> Google fonts/icons © Google. Licensed under Apache 2.0. See `licenses/apache-2.0.txt`.
+
+---
+
+## CSS / JS Libraries (vendored)
+
+- **Bootstrap 4.5.2** — MIT License  
+  **Files:** `public/vendor/bootstrap/4.5.2/bootstrap.min.css`
+
+- **CodeMirror 5.65.5** — MIT License  
+  **Files:** `public/vendor/codemirror/5.65.5/*`
+
+- **DOMPurify 2.4.0** — Apache License 2.0  
+  **Files:** `public/vendor/dompurify/2.4.0/purify.min.js`
+
+- **Fuse.js 6.6.2** — Apache License 2.0  
+  **Files:** `public/vendor/fuse/6.6.2/fuse.min.js`
+
+- **Resumable.js 1.1.0** — MIT License  
+  **Files:** `public/vendor/resumable/1.1.0/resumable.min.js`
+
+> MIT-licensed code: see `licenses/mit.txt`.  
+> Apache-2.0–licensed code: see `licenses/apache-2.0.txt`.
+
+---