feat(home): presence selector as a compact dropdown instead of 3 radios

Replace the three tooltip'd presence radios in the home Friends card with a
pick_list dropdown + a one-line explainer for the current choice, mirroring the
Settings NetworkMode picker. Add PresenceMode::ALL + a Display impl (descriptive
labels) to back the picker. Tightens the Friends card vertically. clippy
--all-targets clean, 256 lib tests green.

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

src/presence.rs

@@ -35,6 +35,10 @@ pub enum PresenceMode {
 }
 
 impl PresenceMode {
+    /// All postures, default first — the option list for the Settings/home picker.
+    pub const ALL: [PresenceMode; 3] =
+        [PresenceMode::Normal, PresenceMode::Invisible, PresenceMode::Discoverable];
+
     /// Whether this posture publishes to discovery (the only mode that does).
     pub fn publishes_to_discovery(self) -> bool {
         matches!(self, PresenceMode::Discoverable)
@@ -46,6 +50,17 @@ impl PresenceMode {
     }
 }
 
+impl std::fmt::Display for PresenceMode {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let label = match self {
+            PresenceMode::Normal => "Normal — friends only",
+            PresenceMode::Invisible => "Invisible — appear offline",
+            PresenceMode::Discoverable => "Discoverable — findable after a move",
+        };
+        f.write_str(label)
+    }
+}
+
 /// What a peer advertises about the gathering they're currently in. Both fields
 /// are peer-supplied and therefore untrusted — see [`interpret_pong`], which
 /// sanitizes the name and validates the ticket before surfacing them.