Add base configuration

- Follow XDG standard: ~/.config and ~/.local folders
- Set environment variables and define aliases within ~/.config/shell
- Add installation script for easy setup and maintenance
  (i.e., `install-dotfiles` and `update-dotfiles`,
   and a `ensure-locales` helper)
- Add README.md with info on the installation and general notes

.local/bin/dotfiles-update

@@ -0,0 +1,87 @@
+#!/bin/sh
+
+# Replace the local `dotfiles` repository with a fresh install
+# by downloading and re-running the latest `dotfiles-install`
+#
+# Refuses to run if there are uncommitted or unpushed changes,
+# unless called with `--force`
+
+set -eu
+
+
+XDG_DATA_HOME="${XDG_DATA_HOME:-$HOME/.local/share}"  # also set in ~/.config/shell/env
+DOTFILES_DIR="$XDG_DATA_HOME/dotfiles"  # also set in ~/.config/shell/env
+
+
+if [ -d "$DOTFILES_DIR" ]; then
+
+    _git() {
+        git --git-dir="$DOTFILES_DIR" --work-tree="$HOME" "$@"
+    }
+
+    # Keep the machine on its current branch (e.g., "desktop")
+    branch=$(_git symbolic-ref --short HEAD) || {
+        echo "" >&2
+        echo "Detached HEAD => re-run after 'dotfiles checkout <branch>'" >&2
+        echo "" >&2
+
+        exit 1
+    }
+
+    [ "${1:-}" = "--force" ] && force=1 || force=0
+
+    if [ "$force" -ne 1 ]; then
+
+        if [ -n "$(_git status --porcelain)" ]; then
+            echo "" >&2
+            echo "There are uncommitted changes!" >&2
+            echo "" >&2
+            _git --no-pager diff HEAD --stat >&2
+            echo "" >&2
+            echo "=> Commit and push them, or re-run with --force" >&2
+            echo "" >&2
+
+            exit 1
+        fi
+
+        if _git rev-parse --verify --quiet "origin/$branch" >/dev/null \
+            && [ -n "$(_git log --oneline "origin/$branch..$branch")" ]; then
+            echo "" >&2
+            echo "There are unpushed commits!" >&2
+            echo "" >&2
+            _git --no-pager log --oneline "origin/$branch..$branch" >&2
+            echo "" >&2
+            echo "=> Push them, or re-run with --force" >&2
+            echo "" >&2
+
+            exit 1
+        fi
+
+    fi
+
+else
+    branch="main"
+fi
+
+
+installer_url="https://code.webartifex.de/alexander/dotfiles/raw/branch/$branch/.local/bin/dotfiles-install"
+
+installer_path=$(mktemp)
+trap 'rm -f "$installer_path"' EXIT
+
+
+if command -v curl >/dev/null 2>&1; then
+    curl -fsSL "$installer_url" -o "$installer_path"
+elif command -v wget >/dev/null 2>&1; then
+    wget -q "$installer_url" -O "$installer_path"
+else
+    echo "" >&2
+    echo "Neither curl nor wget are installed!" >&2
+    echo "=> Install one of them" >&2
+    echo "" >&2
+
+    exit 1
+fi
+
+
+sh "$installer_path" --force