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/rules.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"strings"
 	"unicode"
+	"unicode/utf8"
 )
 
 // Rules are what a key's value must be to share with it: random enough
@@ -40,7 +41,7 @@ func (r Rules) Check(key []byte) error {
 			kinds++
 		}
 	}
-	n := len([]rune(s))
+	n := utf8.RuneCountInString(s)
 	var has, needs []string
 	if n < r.MinLength {
 		has = append(has, fmt.Sprintf("%d characters", n))