feat: add first_run.fish for one-time shell initialization

conf.d/first_run.fish

@@ -0,0 +1,45 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+#          ╭──────────────────────────────────────────────────────────╮
+#          │                First-Run Initialization                  │
+#          ╰──────────────────────────────────────────────────────────╯
+#
+# Runs exactly once on the first interactive fish session after install.
+# To reset for testing, run: set -Ue __fish_config_first_run_complete
+
+# Exit early in non-interactive shells (scripts, completions, subshells)
+if not status is-interactive
+    return
+end
+
+# Skip if this shell has already been initialized
+if set -q __fish_config_first_run_complete
+    return
+end
+
+# Set the flag immediately — before actions — so a mid-run crash doesn't
+# leave the shell in a state that re-triggers everything next session.
+set -U __fish_config_first_run_complete 1
+
+#   ──────────────────────────── Welcome message ───────────────────────────
+echo ""
+echo "  Welcome to your fish shell configuration!"
+echo "  Run 'config-help' for offline documentation."
+echo "  Run 'fish-deps'   to check and install dependencies."
+echo ""
+
+#   ──────────────────────────── Bootstrap Fisher ──────────────────────────
+if not type -q fisher
+    echo "  [first-run] Installing Fisher plugin manager..."
+    if curl -sL https://raw.githubusercontent.com/jorgebucaran/fisher/main/functions/fisher.fish | source
+        and fisher update 2>/dev/null
+        echo "  [first-run] Fisher installed and plugins loaded."
+    else
+        echo "  [first-run] Fisher install failed — run 'fisher update' manually." >&2
+    end
+end
+
+#   ───────────────────────────── Apply theme ──────────────────────────────
+# Catppuccin Mocha theme ships with this config in themes/; it is always available.
+fish_config theme choose "Catppuccin Mocha" 2>/dev/null