feat: drop-in support for custom templates

User templates can now be added without rebuilding: drop a script into
~/.config/motd-assist/templates/ and reference it from a preset via
"install": "<filename>". Same-name files override built-ins; new files
are picked up by the running TUI via 'r'. `motd-assist templates` lists
known templates and creates the drop-in directory.

Apply plans now embed the resolved template content (Plan.ResolveContents)
so the sudo/root process never needs to read user config; snapshots store
installed script content so restore works under sudo as well. Validation
rejects plans with unresolved content.

internal/assets/assets_test.go

@@ -0,0 +1,77 @@
+package assets
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestDropInAndOverride(t *testing.T) {
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+	dir, err := EnsureTemplatesDir()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(dir, "mein-header"),
+		[]byte("#!/bin/sh\necho custom\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(dir, "footer"),
+		[]byte("#!/bin/sh\necho myfooter\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Drop-in wird gefunden.
+	c, ok := ScriptContent("mein-header")
+	if !ok || !strings.Contains(c, "custom") {
+		t.Errorf("ScriptContent(mein-header) = %q, %v", c, ok)
+	}
+
+	// Gleichnamige User-Vorlage überdeckt das Built-in.
+	c, ok = ScriptContent("footer")
+	if !ok || !strings.Contains(c, "myfooter") {
+		t.Errorf("Override greift nicht: %q, %v", c, ok)
+	}
+
+	// Übrige Built-ins bleiben erreichbar.
+	if c, ok = ScriptContent("brand-header"); !ok || !strings.Contains(c, "brand-header") {
+		t.Errorf("Built-in fehlt: %q, %v", c, ok)
+	}
+
+	// Namensliste enthält die Drop-in-Vorlage.
+	var found bool
+	for _, n := range ScriptNames() {
+		if n == "mein-header" {
+			found = true
+		}
+	}
+	if !found {
+		t.Error("mein-header fehlt in ScriptNames()")
+	}
+
+	// MatchScript erkennt Drop-in-Inhalt.
+	name, ok := MatchScript("#!/bin/sh\necho custom\n")
+	if !ok || name != "mein-header" {
+		t.Errorf("MatchScript = %q, %v", name, ok)
+	}
+
+	// Templates() markiert die Quellen richtig.
+	sources := map[string]string{}
+	for _, tpl := range Templates() {
+		sources[tpl.Name] = tpl.Source
+	}
+	if sources["mein-header"] != "user" || sources["footer"] != "user" || sources["brand-header"] != "builtin" {
+		t.Errorf("Quellen falsch: %+v", sources)
+	}
+}
+
+func TestDropInDirMissingIsNoError(t *testing.T) {
+	t.Setenv("XDG_CONFIG_HOME", filepath.Join(t.TempDir(), "leer"))
+	if _, ok := ScriptContent("gibt-es-nicht"); ok {
+		t.Error("unbekannte Vorlage darf nicht gefunden werden")
+	}
+	if len(Templates()) == 0 {
+		t.Error("Built-ins sollten ohne Drop-in-Ordner weiter existieren")
+	}
+}