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/share/share.go

@@ -47,8 +47,8 @@ const (
 	MaxLink = 16 << 10
 	// MinKey is the shortest key the rules can ask for.
 	MinKey = 16
-	// maxFiles caps what opening a link unpacks.
-	maxFiles = 1 << 20
+	// maxUnpacked caps the bytes opening a link unpacks.
+	maxUnpacked = 1 << 20
 )
 
 // workFactor is the scrypt work factor for sealing: 2^15, about 40 ms
@@ -153,6 +153,16 @@ func Seal(h Header, key []byte, files []File, rules Rules) (ref.Ref, error) {
 	return r, nil
 }
 
+// KeyError is a key that can't be used: it can't be read, or doesn't
+// keep to the rules.
+type KeyError struct {
+	Name string // the secret's
+	Err  error
+}
+
+func (e *KeyError) Error() string { return "can't use " + e.Name + ": " + e.Err.Error() }
+func (e *KeyError) Unwrap() error { return e.Err }
+
 // Read reads a share link's header.
 func Read(r ref.Ref) (Sealed, error) {
 	bad := errors.New("share link is cut short or garbled: copy all of it")
@@ -197,11 +207,11 @@ func (s Sealed) Open(key []byte) ([]File, error) {
 		}
 		return nil, fmt.Errorf("can't decrypt the link: %w", err)
 	}
-	b, err := io.ReadAll(io.LimitReader(flate.NewReader(dec), maxFiles+1))
+	b, err := io.ReadAll(io.LimitReader(flate.NewReader(dec), maxUnpacked+1))
 	if err != nil {
 		return nil, fmt.Errorf("can't decrypt the link: %w", err)
 	}
-	if len(b) > maxFiles {
+	if len(b) > maxUnpacked {
 		return nil, errors.New("the link holds more than lazysecrets unpacks")
 	}
 	var plain []file