Tray: copy a network secret via ipc; use text buttons, not missing glyphs

Two fixes.

Secret copy: a new local control request returns a configured network's secret
in its tsn1 text form, so the tray's "Copy secret" button can put it on the
clipboard. The agent reads it from local state (same source as the CLI's
`network secret show`); it crosses only the owner/group socket and is wrapped in
a RedactedText type whose Debug prints "<redacted>", so it can never land in a
log. In the GUI the worker fetches it and hands it to the UI thread (which owns
the clipboard) through a one-shot slot kept out of the status snapshot, so it is
copied once and never stored in the cloned snapshot. Control protocol bumped
16 -> 17.

Icons: the button glyphs (⧉ ⟳ ✎ ✔ ✕) are not in egui's bundled fonts and showed
as tofu squares, so they are now plain text: copy / refresh / random / edit /
save / cancel. No icon font to embed.

Checks: fmt, clippy --workspace --all-targets -D warnings, and the tsunagi lib,
tsunagi-cli and wg-quic local_control tests pass on macOS.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/tsunagi-tray/src/format.rs

@@ -130,7 +130,7 @@ pub(crate) fn copy_label(ui: &mut egui::Ui, display: &str, value: &str) {
 /// A tiny clipboard button that copies `value`.
 pub(crate) fn copy_button(ui: &mut egui::Ui, value: &str) {
     if ui
-        .small_button("⧉")
+        .small_button("copy")
         .on_hover_text("copy to clipboard")
         .clicked()
     {