fix: follow-ups from the post-merge reviews of #6221, #6227, #6230 and #6239 (#6250)

* fix: follow-ups from the post-merge reviews of #6221, #6227, #6230 and #6239

Six defects the automated reviews found after those PRs merged. Each is
verified rather than taken on trust — two by experiment, the rest by
reading the merged code.

**Import restore never wrote an empty local value** (#6227). GORM builds
the assignment map from the struct passed to Assign and drops zero-valued
fields, so `Assign(model.Setting{Value: ""})` produced an empty Updates
and the imported row survived. Empty is the normal state: UpdateAllSetting
writes a row for every AllSetting field including the blank ones. That is
exactly the case the PR existed for — a destination with no certificate
inheriting the source machine's path. Confirmed with a throwaway test
before changing anything: the value stayed "IMPORTED". Now uses
saveSetting, which is not zero-filtered.

**Import destroyed node mTLS material** (#6227). The "no local row means
the default applied, so drop the import" branch fires for the five
nodeMtls* keys, which are minted on demand and deliberately absent from
AllSetting, so a fresh install has no row for them. Reinstall-then-restore
therefore deleted the CA certificate and its private key — and the backup
was the only copy, since neither is surfaced in the UI or the export.
Those keys are now kept.

**The clients-list enable toggle wiped renewal state** (#6239, #6238).
setEnable hand-builds the update payload and carried reset but not
resetDay or resetMax, so one click on the switch turned calendar mode off
and lifted the renewal cap permanently. The form-modal tests could not
catch it because that path does send both fields.

**"Delete depleted clients" deleted calendar clients** (#6239). The
predicate read `reset = 0` as "does not auto-renew", which is exactly the
calendar shape, in two places. Both now share one constant that also
requires `reset_day = 0`.

**Allowlist validation and parsing disagreed** (#6230). Save used net,
scan used netip, and they differ: `198.51.100.0/024` saves without
complaint and is silently dropped at scan — the failure the PR set out to
remove. Verified by running both parsers. An IPv4-mapped prefix parsed but
could never match, because contains() unmaps the query while the prefix
stayed 128-bit; it is unmapped at parse now. A test asserts the two
acceptance sets agree.

**A comment stated the opposite of the truth** (#6221). GetInbounds has no
enable filter, so a node reports a disabled inbound normally; the row in
that bug report was missing only because it was never delivered. Reworded
to the real invariant.

Also trims two comment blocks in ip_limit_allowlist.go to the repo's
two-line maximum.

Not included: the reviewer's suggestion to lift the node hand-off out of
`if inbound.Enable` in AddInbound. It is the right root-cause fix, but it
changes delivery behaviour on multi-node deployments and belongs in its
own change with its own testing, not in a cleanup batch.

One reported finding is not real: BulkCreate does call
validateClientResetDay, validateClientResetMax and
validateClientTrafficReset — verified in the merged tree.

* fix(netsafe): wrap both errors so errorlint passes

Unrelated to this PR's subject and in a file it does not otherwise touch.
It is here only because CI lints the merge result, and `main` has been red
since #6242 landed: `fmt.Errorf("%w; %v", ...)` wraps the first error and
formats the second, which errorlint rejects. Go 1.20 allows more than one
%w, so both are wrapped now and `errors.Is` works against either.

internal/web/service/server.go

@@ -1488,25 +1488,37 @@ func restoreHostBoundSettings(snap hostBoundSnapshot) {
 	if db == nil {
 		return
 	}
+	settingSvc := &SettingService{}
 	for _, key := range hostBoundSettingKeys {
 		if _, had := snap.present[key]; !had {
-			// No row here before the import, so the default applied. Drop the
-			// imported row rather than inherit the source machine's value.
+			// Absent because it is minted on demand, not because a default applied:
+			// the imported copy is the only one that exists, so keep it (#6227).
+			if lazilyMintedSettingKeys[key] {
+				continue
+			}
 			if err := db.Where("key = ?", key).Delete(&model.Setting{}).Error; err != nil {
 				logger.Warningf("Import: could not drop imported setting %q: %v", key, err)
 			}
 			continue
 		}
-		// The imported row may or may not exist; settings are key-value, so an
-		// upsert keyed on the name is the only safe write here.
-		if err := db.Where(model.Setting{Key: key}).
-			Assign(model.Setting{Value: snap.values[key]}).
-			FirstOrCreate(&model.Setting{}).Error; err != nil {
+		// saveSetting rather than Assign(struct): GORM drops zero-valued fields from
+		// the assignment map, so an empty local value never overwrote the import.
+		if err := settingSvc.saveSetting(key, snap.values[key]); err != nil {
 			logger.Warningf("Import: could not restore setting %q for this machine: %v", key, err)
 		}
 	}
 }
 
+// Minted on demand, so a fresh install has no row: dropping the imported copy
+// would destroy the only one that exists, CA private key included.
+var lazilyMintedSettingKeys = map[string]bool{
+	"nodeMtlsCaCertPem":     true,
+	"nodeMtlsCaKeyPem":      true,
+	"nodeMtlsClientCertPem": true,
+	"nodeMtlsClientKeyPem":  true,
+	"nodeMtlsClientCAPem":   true,
+}
+
 func (s *ServerService) ImportDB(file multipart.File, keepHostSettings bool) error {
 	if database.IsPostgres() {
 		return s.importPostgresDB(file, keepHostSettings)