refactor: tidy up sharing after the round of tweaks

- Go to's picker settings are one picker struct, set while picking.
- Opening a share link plans the save, then asks or saves; no more
  offerWrite doing either by a flag.
- Closing the picker while it encrypts cancels the share, and a
  second pick meanwhile is ignored.
- Favourites and Recent leave out plain env values as keys too.
- share.key in the config is checked with share.ParseKey, so
  {{secrets/…}} is refused there already.
- A key that can't be read reads the same opening as sharing
  ("can't use …"); KeyError moved to share.go, maxFiles is
  maxUnpacked (bytes), and a stray comment is gone.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

internal/ui/app/jump.go

@@ -47,6 +47,7 @@ func (m *Model) storeEntries() (favs, recent []option) {
 			out = append(out, entry{depth, sort, option{
 				label: m.pathLabel(r.Kind, path, account), note: n.Note, mark: " ",
 				id: prefix + key + "|" + acct, star: key, starAccount: acct, starred: n.StarredFor(acct),
+				plain: m.inPlainEnv(r),
 				run: func(m *Model) tea.Cmd {
 					m.used(key)
 					return run(m)
@@ -138,6 +139,24 @@ func (m Model) pathLabel(kind provider.Kind, path []string, account string) stri
 	return label
 }
 
+// inPlainEnv reports whether r is a key in a plain env file, as far as
+// the files loaded tell.
+func (m Model) inPlainEnv(r ref.Ref) bool {
+	if r.Kind != provider.KindRepo || r.Name == "" {
+		return false
+	}
+	file := r
+	file.Name, file.Type = "", ""
+	for _, e := range m.cache.containers {
+		for _, c := range e.value {
+			if c.Detail == provider.DetailPlainEnv && storeKey(c) == file.String() {
+				return true
+			}
+		}
+	}
+	return false
+}
+
 // repoLabel draws a repo, or a file in it, as everywhere else: the repo
 // icon and colour, "app / apps/api/.env".
 func (m Model) repoLabel(repo string, file ...string) string {