system updater: complete overhaul

Move system update flow to GO, with a CLI (convenient AIO tool) and
server integration. All lifecycle, scheduling, execution occurs on
backend side.

Run some backends via pkexec, some via terminal like paru/yay.

Incorporate flatpak as an option to update.

Add terminal override setting in GUI, in addition to $TERMINAL env
variable.

fixes #2307
fixes #822
fixes #1102
fixes #1812
fixes #1087
fixes #1743

core/internal/server/sysupdate/backend.go

@@ -0,0 +1,96 @@
+package sysupdate
+
+import (
+	"context"
+	"os/exec"
+	"sync"
+)
+
+type Backend interface {
+	ID() string
+	DisplayName() string
+	Repo() RepoKind
+	IsAvailable(ctx context.Context) bool
+	NeedsAuth() bool
+	RunsInTerminal() bool
+	CheckUpdates(ctx context.Context) ([]Package, error)
+	Upgrade(ctx context.Context, opts UpgradeOptions, onLine func(string)) error
+}
+
+type Selection struct {
+	System  Backend
+	Overlay []Backend
+}
+
+func (s Selection) All() []Backend {
+	if s.System == nil {
+		return s.Overlay
+	}
+	out := make([]Backend, 0, 1+len(s.Overlay))
+	out = append(out, s.System)
+	out = append(out, s.Overlay...)
+	return out
+}
+
+func (s Selection) Info() []BackendInfo {
+	all := s.All()
+	out := make([]BackendInfo, 0, len(all))
+	for _, b := range all {
+		out = append(out, BackendInfo{
+			ID:             b.ID(),
+			DisplayName:    b.DisplayName(),
+			Repo:           b.Repo(),
+			NeedsAuth:      b.NeedsAuth(),
+			RunsInTerminal: b.RunsInTerminal(),
+		})
+	}
+	return out
+}
+
+var (
+	registryMu       sync.RWMutex
+	systemCandidates []func() Backend
+	overlayCandidate []func() Backend
+)
+
+func RegisterSystemBackend(factory func() Backend) {
+	registryMu.Lock()
+	defer registryMu.Unlock()
+	systemCandidates = append(systemCandidates, factory)
+}
+
+func RegisterOverlayBackend(factory func() Backend) {
+	registryMu.Lock()
+	defer registryMu.Unlock()
+	overlayCandidate = append(overlayCandidate, factory)
+}
+
+func Select(ctx context.Context) Selection {
+	registryMu.RLock()
+	sys := append([]func() Backend(nil), systemCandidates...)
+	ov := append([]func() Backend(nil), overlayCandidate...)
+	registryMu.RUnlock()
+
+	var sel Selection
+	for _, factory := range sys {
+		b := factory()
+		if !b.IsAvailable(ctx) {
+			continue
+		}
+		sel.System = b
+		break
+	}
+	for _, factory := range ov {
+		b := factory()
+		if !b.IsAvailable(ctx) {
+			continue
+		}
+		sel.Overlay = append(sel.Overlay, b)
+	}
+	return sel
+}
+
+func commandExists(name string) bool {
+	_, err := exec.LookPath(name)
+	return err == nil
+}