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/links.go

@@ -818,7 +818,7 @@ func (m *Model) openText(s string) tea.Cmd {
 
 // handlePaste opens a pasted reference, unless a text input has focus.
 func (m *Model) handlePaste(msg tea.PasteMsg) tea.Cmd {
-	if m.search != nil && m.search.pick != nil {
+	if m.search != nil && m.search.picker != nil {
 		return m.pickPasted(msg.Content) // a reference to the secret
 	}
 	if p := m.prompt; p != nil && p.input != nil {