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/ui/update.go

@@ -249,6 +249,12 @@ func (m Model) applyWorking() (tea.Model, tea.Cmd) {
 		m.status = "Keine Änderungen zum Anwenden"
 		return m, nil
 	}
+	// Vorlagen-Inhalte jetzt auflösen (User-Templates liegen im eigenen
+	// $HOME); der sudo-Prozess kann sie später nicht mehr lesen.
+	if err := plan.ResolveContents(); err != nil {
+		m.status = err.Error()
+		return m, nil
+	}
 	m.applying = true
 	m.lastSummary = plan.Summary()
 	return m, applyExec(plan)