Default the update repo to the Gitea host

Settles the first open question in claude.md: the update endpoint is
gitea.apointless.space/bsncubed/ipswap. It was left empty — which disables the
checker — only because the host was unconfirmed.

Load unmarshals over the defaults, so an absent update_repo takes the new
default while an explicitly empty one stays empty. That distinction is the off
switch, and forks depend on it, so it is now pinned by a test.

Adds the config package's first tests while here: the defaulting rules above,
the repair path for a nonsensical mask style or poll interval, and the
guarantee that a corrupt config.json still yields usable defaults rather than
stopping the app from starting.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/config/config.go

@@ -64,7 +64,7 @@ type Settings struct {
 
 	// UpdateRepo is the Gitea repo polled for releases, as
 	// "https://host/owner/repo". Empty disables the update checker entirely,
-	// which is the default until the host is confirmed.
+	// which is what a fork or a private build should set it to.
 	UpdateRepo string `json:"update_repo"`
 
 	// ActivePollSeconds is how often the live adapter state is re-read to work
@@ -73,12 +73,16 @@ type Settings struct {
 }
 
 // Default is the settings a fresh install starts with.
+// DefaultUpdateRepo is where release builds look for updates. Set it to "" in
+// config.json (or clear the field in Settings) to turn the check off entirely.
+const DefaultUpdateRepo = "https://gitea.apointless.space/bsncubed/ipswap"
+
 func Default() Settings {
 	return Settings{
 		MaskStyle:         MaskPrefix,
 		StartWithWindows:  false,
 		CheckUpdates:      true,
-		UpdateRepo:        "",
+		UpdateRepo:        DefaultUpdateRepo,
 		ActivePollSeconds: 30,
 	}
 }