refactor: drop dead code and use the standard library where it already does the job

No behaviour change.

- Deleted, unused or one-line wrappers: panes.RuneIndexes, Detail.Refresh (SetDescribed does it), SecretValue.Len, Query.Empty, Clipboard.Native, config.Load, get's value, Model.done, providerOf (cachefile.ProviderOf), contextWithTimeout, searchCoverage, startCrawl, showKeybindings, the expiry aliases and daysLeft.
- cmp.Or instead of the two cmpOr copies; slices.Sorted(maps.Keys(…)), maps.Copy, wg.Go, utf8.RuneCountInString and new(true) instead of loops and helpers that do the same.
- A container's pane title is set once, not twice; a redundant length check in share links and a no-op assignment in linkhandler.Launch are gone.
- Tests call ansi.Strip and lastLines directly; gofmt -s on links.go.

internal/linkhandler/install_darwin.go

@@ -116,17 +116,13 @@ func Check() string {
 	if err != nil {
 		return "" // installed before the key was recorded: assume it works
 	}
-	if exe := strings.TrimSpace(string(out)); !exists(exe) {
+	exe := strings.TrimSpace(string(out))
+	if _, err := os.Stat(exe); err != nil {
 		return "lazysecrets:// links run " + exe + ", which no longer exists"
 	}
 	return ""
 }
 
-func exists(path string) bool {
-	_, err := os.Stat(path)
-	return err == nil
-}
-
 // Uninstall removes the applet and its registration.
 func Uninstall() (string, error) {
 	app, err := AppPath()