feat: a key that can't be used is explained in the picker

The key picker stays open until the link is made, saying "encrypting
with …" meanwhile. A key that can't be read, or doesn't keep to
share.keyRules, is shown in it in red with which rule it misses
("its value has 28 characters; share.keyRules needs 32
(minLength)"), to pick another.

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

internal/ui/app/sharefiles.go

@@ -137,8 +137,14 @@ func (m *Model) confirmShare(s sharing) {
 func (m *Model) pickShareKey(s sharing) tea.Cmd {
 	cmd := m.openGoTo(tabSearch, true)
 	m.search.pick = func(m *Model, key ref.Ref) tea.Cmd {
+		// The picker showed what's shared: no need to ask again. It
+		// stays open until the link is made, to say why a key can't be
+		// used.
 		s.key = key
-		return m.seal(s) // the picker showed what's shared
+		m.search.pickErr, m.search.pickBusy = "", "encrypting with "+key.Name+"…"
+		cmd := m.seal(s)
+		m.flash = flash{}
+		return cmd
 	}
 	m.search.pickTitle = shareTitle(s)
 	m.search.pickLines = m.sharedFiles(s)
@@ -159,9 +165,7 @@ func (m *Model) pickPasted(text string) tea.Cmd {
 		m.flash = flash{text: "share: " + err.Error(), isErr: true}
 		return nil
 	}
-	pick := m.search.pick
-	m.search = nil
-	return pick(m, key)
+	return m.search.pick(m, key)
 }
 
 // seal reads the key and encrypts the files with it.
@@ -171,7 +175,7 @@ func (m *Model) seal(s sharing) tea.Cmd {
 	return func() tea.Msg {
 		v, version, err := read(s.key)
 		if err != nil {
-			return sealedMsg{err: fmt.Errorf("reading the key %s: %w", s.key.Name, err)}
+			return sealedMsg{err: &share.KeyError{Name: s.key.Name, Err: err}}
 		}
 		link, err := share.Seal(share.Header{Key: share.Pin(s.key, version), Repo: s.repo.Detail}, []byte(v.Reveal()), s.files, rules)
 		return sealedMsg{link: link, n: len(s.files), err: err}
@@ -179,11 +183,24 @@ func (m *Model) seal(s sharing) tea.Cmd {
 }
 
 func (m *Model) handleSealed(msg sealedMsg) tea.Cmd {
-	if msg.err != nil {
+	picking := m.search != nil && m.search.pick != nil
+	if picking {
+		m.search.pickBusy = ""
+	}
+	var bad *share.KeyError
+	switch {
+	case msg.err != nil && picking && errors.As(msg.err, &bad):
+		// Picked: say why in the picker, to pick another.
+		m.search.pickErr = "✗ " + strings.ToUpper(bad.Error()[:1]) + bad.Error()[1:]
+		return nil
+	case msg.err != nil:
 		m.noteErr(msg.err)
 		m.flash = flash{text: "share: " + msg.err.Error(), isErr: true}
 		return nil
 	}
+	if picking {
+		m.search = nil
+	}
 	m.dismissedLink = msg.link.String() // not offered back on focus
 	return m.copyText("encrypted link to "+plural(msg.n, "file"), msg.link.String(), false)
 }