refactor: move build scripts to build/ directory

Declutters the repo root by consolidating build-common.sh,
build-dist.sh, build-dist-windows.sh, build-dist.ps1, and installer.nsi
into build/. Updates all path references in CI workflows, NSIS installer,
and documentation.

.gitea/workflows/release.yml

@@ -135,8 +135,8 @@ jobs:
 
       - name: Cross-build Windows distribution
         run: |
-          chmod +x build-dist-windows.sh
-          ./build-dist-windows.sh "${{ gitea.ref_name }}"
+          chmod +x build/build-dist-windows.sh
+          ./build/build-dist-windows.sh "${{ gitea.ref_name }}"
 
       - name: Upload build artifacts
         uses: actions/upload-artifact@v3
@@ -207,8 +207,8 @@ jobs:
 
       - name: Build Linux distribution
         run: |
-          chmod +x build-dist.sh
-          ./build-dist.sh "${{ gitea.ref_name }}"
+          chmod +x build/build-dist.sh
+          ./build/build-dist.sh "${{ gitea.ref_name }}"
 
       - name: Upload build artifact
         uses: actions/upload-artifact@v3