feat(zsh): replace fnm with standalone pnpm

Install the pnpm native binary from GitHub releases via gh-r and sbin,
matching the existing fnm/starship shim pattern. Node is now managed
by pnpm itself (pnpm runtime), so fnm and its init block are removed.

Drop unzip from the README optional list; only fnm's zip needed it.

zsh/.config/zsh/.zshrc

@@ -39,26 +39,12 @@ zinit for \
     lbin'!' \
     @ajeetdsouza/zoxide
 
-# pick the correct fnm release asset for this OS/arch
-if [[ "$OSTYPE" == darwin* ]]; then
-    _fnm_asset='fnm-macos.zip'
-elif [[ "$(uname -m)" == (aarch64|arm64) ]]; then
-    _fnm_asset='fnm-arm64.zip'
-else
-    _fnm_asset='fnm-linux.zip'
-fi
+
 zinit for \
     id-as \
     from'gh-r' \
-    bpick"$_fnm_asset" \
-    atclone'
-        ./fnm env --use-on-cd --shell zsh > init.zsh; \
-        ./fnm completions --shell zsh > _fnm;
-    ' \
-    atpull'%atclone' \
-    blockf \
-    sbin'fnm' \
-    @Schniz/fnm
+    sbin'pnpm' \
+    @pnpm/pnpm
 
 zinit for \
     id-as \