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/share/share.go

@@ -174,7 +174,7 @@ func Read(r ref.Ref) (Sealed, error) {
 		return Sealed{}, errors.New("share link is from a newer lazysecrets: update it (lazysecrets update)")
 	}
 	n, size := binary.Uvarint(b[1:])
-	if size <= 0 || n > uint64(len(b)) {
+	if size <= 0 {
 		return Sealed{}, bad
 	}
 	rest := b[1+size:]