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/js/main.js

@@ -26,6 +26,30 @@ let deferredInstallPrompt = null;
 let lotPicker = null;
 const BAND_DOMAIN = [60, 220]; // shared °C domain for every band-track in the Machine Plan section
 
+// Temperature fields are always stored in state.plan as canonical °C — this is purely a
+// display-layer preference. tempC/actualBt/expectedBt cover Machine Plan, Roast Log, and
+// the actuator schedule respectively; every other numeric field (%, g, days) is untouched.
+const TEMP_UNIT_KEY = "roastPlannerTempUnit.v1";
+const TEMP_FIELD_RE = /\.(tempC|actualBt|expectedBt)$/;
+let tempUnit = localStorage.getItem(TEMP_UNIT_KEY) === "F" ? "F" : "C";
+const cToF = (c) => (c * 9) / 5 + 32;
+const fToC = (f) => ((f - 32) * 5) / 9;
+function isTempField(name) {
+	return TEMP_FIELD_RE.test(name);
+}
+// Canonical °C string -> what the field should display right now.
+function tempToDisplay(canonical) {
+	const n = Number.parseFloat(canonical);
+	if (!Number.isFinite(n)) return canonical ?? "";
+	return String(Math.round(tempUnit === "F" ? cToF(n) : n));
+}
+// What the user typed (in the current display unit) -> canonical °C string.
+function tempToCanonical(display) {
+	const n = Number.parseFloat(display);
+	if (!Number.isFinite(n)) return display;
+	return String(Math.round((tempUnit === "F" ? fToC(n) : n) * 10) / 10);
+}
+
 export const state = { plan: blankPlan() };
 
 const form = document.getElementById("plan-form");
@@ -201,6 +225,7 @@ function renderActuatorTimeline() {
 function renderActuators() {
 	renderActuatorPrintRows();
 	renderActuatorTimeline();
+	updateTempUnitUI(); // new actuator rows need their expectedBt placeholder set too
 }
 
 // ---- populate every control in the form from state.plan
@@ -214,7 +239,7 @@ export function renderFormFromPlan() {
 		} else if (el.type === "checkbox") {
 			el.checked = Boolean(value);
 		} else {
-			el.value = value ?? "";
+			el.value = isTempField(name) ? tempToDisplay(value) : (value ?? "");
 		}
 	}
 	updateBlendVisibility(state.plan.fields["2.1"]);
@@ -310,7 +335,8 @@ function renderBandMarkers() {
 		const input = row.querySelector('input[name$=".tempC"]');
 		const marker = row.querySelector(".band-marker");
 		if (!input || !marker) continue;
-		const v = Number.parseFloat(input.value);
+		const raw = Number.parseFloat(input.value);
+		const v = Number.isFinite(raw) ? (tempUnit === "F" ? fToC(raw) : raw) : NaN;
 		if (!Number.isFinite(v)) {
 			marker.style.display = "none";
 			continue;
@@ -587,7 +613,10 @@ function wireForm() {
 		const el = e.target;
 		if (!el.name) return;
 		if (el.type === "radio" && !el.checked) return;
-		setValueForName(el.name, el.value);
+		setValueForName(
+			el.name,
+			isTempField(el.name) ? tempToCanonical(el.value) : el.value,
+		);
 		if (el.name === "1.1") cultivarAutofill(el.value);
 		if (el.name === "2.1") updateBlendVisibility(el.value);
 		if (el.name.startsWith("blendComponents.")) updateBlendTotal();
@@ -595,6 +624,38 @@ function wireForm() {
 	});
 }
 
+// Refreshes every unit-facing bit of chrome (labels, placeholders, the toggle itself) after
+// tempUnit changes. Field *values* are handled separately by renderFormFromPlan().
+function updateTempUnitUI() {
+	const label = tempUnit === "F" ? "°F" : "°C";
+	for (const input of document.querySelectorAll(
+		'input[name$=".tempC"], input[name$=".actualBt"], input[name$=".expectedBt"]',
+	))
+		input.placeholder = label;
+	for (const span of document.querySelectorAll("#sec-machine .unit-input .unit"))
+		span.textContent = label;
+	const printHeader = document.getElementById("print-machine-temp-th");
+	if (printHeader) printHeader.textContent = label;
+	for (const input of document.querySelectorAll('#temp-unit-toggle input[name="tempUnit"]'))
+		input.checked = input.value === tempUnit;
+}
+
+function setTempUnit(unit) {
+	tempUnit = unit === "F" ? "F" : "C";
+	localStorage.setItem(TEMP_UNIT_KEY, tempUnit);
+	updateTempUnitUI();
+	renderFormFromPlan();
+	recompute();
+}
+
+function wireTempUnitToggle() {
+	const toggle = document.getElementById("temp-unit-toggle");
+	if (!toggle) return;
+	toggle.addEventListener("change", (e) => {
+		if (e.target.name === "tempUnit") setTempUnit(e.target.value);
+	});
+}
+
 function wireDrawers() {
 	const overlay = document.getElementById("drawer-overlay");
 	const drawers = [...document.querySelectorAll(".drawer")];
@@ -941,6 +1002,8 @@ async function init() {
 	initSideNav();
 	wireDrawers();
 	wireToolbar();
+	wireTempUnitToggle();
+	updateTempUnitUI();
 	wirePwa();
 	wireSectionNav();
 	initPrefillPanel({