Replace starship with custom fish powerline prompt

Starship can't color individual path segments separately. Custom fish
prompt with proper Unicode powerline glyphs (U+E0B0 arrows, U+E0B6
left cap). Each folder gets its own colored segment cycling through
teal/green/blue, with chevron transitions between all segments.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/fastfetch.nix

@@ -12,9 +12,8 @@
     nerd-fonts.meslo-lg
   ];
 
-  # Simple fastfetch config — shown on terminal start
-  # Run `fastfetch` manually for full system info
-  environment.etc."fastfetch/config.jsonc".text = ''
+  # Simple fastfetch config — run `fastfetch` manually for system info
+  environment.etc."fastfetch/config.jsonc".text = ''''''
     {
       "$schema": "https://github.com/fastfetch-cli/fastfetch/raw/dev/doc/json_schema.json",
       "logo": {
@@ -44,76 +43,88 @@
         "memory"
       ]
     }
-  '';
+  '''''';
 
-  # Starship cross-shell prompt
-  programs.starship = {
-    enable = true;
-    settings = {
-      "$schema" = "https://starship.rs/config-schema.json";
-      format = builtins.concatStringsSep "" [
-        "[](fg:#394b70)"
-        "$os"
-        "[](bg:#e0af68 fg:#394b70)"
-        "$hostname"
-        "[](bg:#41a6b5 fg:#e0af68)"
-        "$directory"
-        "[](fg:#41a6b5 bg:#bb9af7)"
-        "$git_branch"
-        "$git_status"
-        "[](fg:#bb9af7)"
-        "$nix_shell"
-        "\n"
-        "$character"
-      ];
-
-      os = {
-        style = "bg:#394b70 fg:#7dcfff";
-        format = "[$symbol ]($style)";
-        disabled = false;
-        symbols.NixOS = " ";
-      };
-
-      hostname = {
-        ssh_only = false;
-        style = "bg:#e0af68 fg:#1a1b26";
-        format = "[ $hostname ]($style)";
-      };
-
-      directory = {
-        style = "bg:#41a6b5 fg:#1a1b26";
-        format = "[ $path ]($style)";
-        truncation_length = 4;
-        truncation_symbol = ".../";
-      };
-
-      git_branch = {
-        symbol = "";
-        style = "bg:#bb9af7 fg:#1a1b26";
-        format = "[ $symbol $branch ]($style)";
-      };
-
-      git_status = {
-        style = "bg:#bb9af7 fg:#1a1b26";
-        format = "[$all_status$ahead_behind ]($style)";
-      };
-
-      nix_shell = {
-        symbol = " ";
-        style = "bold yellow";
-        format = " [$symbol$state]($style)";
-      };
-
-      character = {
-        success_symbol = "[❯](bold purple)";
-        error_symbol = "[❯](bold red)";
-      };
-    };
-  };
-
-  # Fish shell settings
-  programs.fish.interactiveShellInit = ''
+  # Fish shell settings and custom powerline prompt
+  programs.fish.interactiveShellInit = ''''''
     # Disable default greeting
     set -g fish_greeting
-  '';
+
+    # Custom powerline prompt
+    function fish_prompt
+      set -l last_status $status
+
+      # Nix-shell indicator
+      if set -q IN_NIX_SHELL
+        set_color -o yellow
+        printf "[nix-shell] "
+        set_color normal
+      end
+
+      # --- Line 1: powerline segments ---
+      # Left round cap + NixOS icon
+      set_color #394b70
+      printf ""
+      set_color #7dcfff -b #394b70
+      printf "  "
+
+      # Arrow transition: nix -> hostname
+      set_color #394b70 -b #e0af68
+      printf ""
+      set_color #1a1b26 -b #e0af68
+      printf " %s " (hostname)
+
+      # Path segments - each folder gets its own color
+      set -l realhome (string escape --style=regex -- $HOME)
+      set -l rawpath (string replace -r "^$realhome" "~" $PWD)
+      set -l parts (string split "/" $rawpath)
+      set -l path_colors #41a6b5 #9ece6a #7aa2f7
+
+      set -l prev_bg #e0af68
+      set -l seg_count 0
+
+      for part in $parts
+        if test -n "$part"
+          set seg_count (math $seg_count + 1)
+          set -l cidx (math "(" $seg_count - 1 ")" "%" 3 + 1)
+          set -l seg_bg $path_colors[$cidx]
+
+          # Arrow from previous segment
+          set_color $prev_bg -b $seg_bg
+          printf ""
+          set_color #1a1b26 -b $seg_bg
+          printf " %s " $part
+
+          set prev_bg $seg_bg
+        end
+      end
+
+      # Git branch (if in a repo)
+      set -l branch (command git branch --show-current 2>/dev/null)
+      if test -n "$branch"
+        set_color $prev_bg -b #bb9af7
+        printf ""
+        set_color #1a1b26 -b #bb9af7
+        printf "  %s " $branch
+        set prev_bg #bb9af7
+      end
+
+      # Final arrow to terminal bg
+      set_color $prev_bg -b normal
+      printf ""
+      set_color normal
+      printf "\n"
+
+      # --- Line 2: prompt character ---
+      if test $last_status -ne 0
+        set_color red
+      else
+        set_color magenta
+      end
+      printf "❯ "
+      set_color normal
+    end
+
+    function fish_right_prompt; end
+  '''''';
 }