Add Fyne GUI and portable --data-dir mode to nymdrop-reader

Extract reader logic (key management, embedded nym-client lifecycle,
receive loop, decrypt, save) into internal/reader as a Hooks-driven
package, shared by both the existing headless CLI and a new --gui mode
built with Fyne. Add --data-dir to make a reader instance fully
self-contained: privkey, inbox, and the embedded nym-client's own state
(normally fixed to $HOME/.nym) all live under the given directory via a
HOME override on the subprocess, enabling zero-trace USB operation.

CLI default behavior is unchanged (same paths, same protocol) so the
pietro deployment is unaffected.

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