Add a °C/°F display toggle for Machine Plan and Roast Log temps

Plans are always stored in canonical °C; the unit is a client-side
display preference (localStorage) that converts Machine Plan planned
temps, Roast Log actual temps, and actuator expected BT on the fly,
including band-marker positioning. Toggle lives in Settings & sync.

public/index.html

@@ -310,6 +310,30 @@
           saved locally first, then synced to your account whenever you are
           online.
         </p>
+        <div class="field">
+          <span class="field-label">Temperature unit</span>
+          <div
+            class="segmented small"
+            id="temp-unit-toggle"
+            role="radiogroup"
+            aria-label="Temperature unit"
+          >
+            <label class="seg-opt"
+              ><input type="radio" name="tempUnit" value="C" checked /><span
+                >°C</span
+              ></label
+            >
+            <label class="seg-opt"
+              ><input type="radio" name="tempUnit" value="F" /><span
+                >°F</span
+              ></label
+            >
+          </div>
+          <p class="field-note">
+            Changes how Machine Plan and Roast Log temperatures are shown.
+            Plans are always stored in °C, so switching back is lossless.
+          </p>
+        </div>
         <p class="drawer-result">
           Install Roast Planner from your browser menu for a focused workspace.
           Updates wait for your confirmation so an in-progress plan is never
@@ -1979,7 +2003,7 @@
                     <tr>
                       <th style="width: 28mm">Milestone</th>
                       <th class="num" style="width: 20mm">Time</th>
-                      <th class="num" style="width: 18mm">°C</th>
+                      <th class="num" style="width: 18mm" id="print-machine-temp-th">°C</th>
                       <th class="num" style="width: 36mm">Your logged band</th>
                     </tr>
                   </thead>