flo-bui: show the tracked distance in the phone view (Claude Opus 5)

Follow-up to the phone view. Standing next to FLO, the one number worth having
is how far away the thing it is tracking is: it says whether FLO has a subject
at a plausible range at all, and it is what the focus follows.

Shown at the readout size the full UI's Info block uses, and formatted the same
way, rather than as another line of small text beside the mode -- it is read at
arm's length. An em dash when there is no estimate, which is a monocular
deployment or no pair of detections yet. Deliberately the stereopsis
measurement and not the home position's distance, which is a setting.

Checked in a browser against the hardware-free simulation, with the synthetic
source sweeping the distance: the readout follows it (1.31m, 2.00m, 2.40m) and
agrees with the full UI's Info block down to the formatting.

The frame counter asked for alongside this is not here: the live view is
strand-braid's `CamPreview`, which knows each frame's number but hands its host
nothing, so a counter needs either a prop added there or a preview component of
our own. Left for a separate decision.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: astraw

README.md

@@ -199,6 +199,8 @@ standing next to FLO and nothing else:
   tracking cameras' MP4s beside it. That checkbox is the same setting the full
   UI carries and defaults to on.
 - **Set Home**, **Go Home** and **Track**.
+- The stereopsis **Distance**, or an em dash when there is no estimate — a
+  monocular deployment, or no pair of detections yet.
 - A small live view of the main tracking camera.
 - The current mode, with the same liveness dot the full UI's Info block has.
 

crates/flo-bui/src/mobile.rs

@@ -42,11 +42,38 @@ impl App {
                         <Button title="Track" onsignal={ctx.link().callback(|_| Msg::SwitchToClosedLoop)}/>
                     </div>
                 </div>
+                { self.mobile_distance() }
                 { self.mobile_preview() }
             </div>
         }
     }
 
+    /// How far away the thing being tracked is, as stereopsis measures it.
+    ///
+    /// The one number worth having out here: it says whether FLO has a subject
+    /// at a plausible range, and it is what the focus follows. Shown at the
+    /// readout size the full UI uses rather than as another line of small text,
+    /// because it is read at arm's length.
+    ///
+    /// An em dash when there is no estimate — a monocular deployment, or no
+    /// pair of detections yet. Deliberately not the home position's distance,
+    /// which is a setting rather than a measurement.
+    fn mobile_distance(&self) -> Html {
+        let distance = match self
+            .last_state
+            .as_ref()
+            .and_then(|state| state.stereopsis_state.as_ref())
+        {
+            Some(stereopsis) => format!("{:.2}m", stereopsis.dist),
+            None => crate::NO_DATA.to_string(),
+        };
+        html! {
+            <div class="border-1px">
+                { crate::qqblock("Distance", distance) }
+            </div>
+        }
+    }
+
     /// The one readout the phone view keeps: which mode FLO is in, beside the
     /// liveness dot.
     ///