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/rules.go

@@ -67,13 +67,3 @@ func (r Rules) withDefaults() Rules {
 	r.MinLength = max(r.MinLength, MinKey)
 	return r
 }
-
-// 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 }