Render protocol priority buttons with Phosphor icons

The tray protocol selection used ordinary Unicode up/down arrows for its
priority buttons. The default UI fonts lack these glyphs, so both buttons
rendered missing-glyph boxes. Use the regular Phosphor ARROW_UP and ARROW_DOWN
icons already installed in the application's proportional font fallback.

Validation on Linux through nix develop:
- cargo fmt --all -- --check
- cargo clippy --locked --workspace --all-targets -- -D warnings
- cargo test --locked --workspace --all-targets, with inherited control socket
  and group overrides removed: 509 passed, 2 existing ignored tests

No new tests were added for the two glyph substitutions. Manual tray rendering
and macOS/Windows checks were not performed. No release tag was requested for
this fix.

crates/tsunagi-tray/src/ui.rs

@@ -390,7 +390,7 @@ fn draw_protocols(
                         }
                         if let Some(index) = selected.iter().position(|p| p == &protocol.name) {
                             if ui
-                                .add_enabled(index > 0, egui::Button::new("↑"))
+                                .add_enabled(index > 0, egui::Button::new(icon::ARROW_UP))
                                 .on_hover_text("prefer earlier")
                                 .clicked()
                             {
@@ -398,7 +398,10 @@ fn draw_protocols(
                                 changed = true;
                             }
                             if ui
-                                .add_enabled(index + 1 < selected.len(), egui::Button::new("↓"))
+                                .add_enabled(
+                                    index + 1 < selected.len(),
+                                    egui::Button::new(icon::ARROW_DOWN),
+                                )
                                 .on_hover_text("prefer later")
                                 .clicked()
                             {