web: an FX chip that flips the photo to monochrome

It is a switch, not a look: it swaps the base filter for the mono stock and
swaps right back to the one the photo was wearing — name and sim included. The
adjustments are never touched, so a knob moved while it is on survives the trip
back, and the swap rides the undo stack like any other edit.

docker/frontend/src/App.tsx

@@ -256,6 +256,9 @@ export function Workspace() {
   // Which look of the open frame's own history is standing on the stage, if one
   // was picked from its column; a fresh edit or another frame clears it.
   const [histPick, setHistPick] = useState<number | null>(null);
+  // FX's MONOCHROME switch remembers what the photo was wearing, so flipping it
+  // back returns that filter — and its sim name — instead of guessing PROVIA.
+  const monoPrev = useRef<{ baseFilter: Recipe['baseFilter']; simId: string | null; name: string } | null>(null);
   const lookRef = useRef<Look | null>(null);
   lookRef.current = { recipe, simId, frameId, crop, cropRect, cropApplied, rotation, markOn, markText, markStyle, useGeotag, gpsStyle };
   // Twenty looks is enough to walk, but the oldest entry is the floor — the look
@@ -1331,6 +1334,23 @@ export function Workspace() {
   // looks it carried before its last saves and its landing-strip consent.
   const openPhoto = myPhotos.find((p) => p.id === openPhotoId);
 
+  // FX's MONOCHROME switch is not a look of its own: it swaps the base filter
+  // for the mono stock and swaps right back. Adjustments are never touched, so
+  // a knob moved while the switch is on survives the trip back.
+  const monoOn = recipe.baseFilter === 'monochrome';
+  const toggleMono = () => {
+    remember();
+    if (monoOn) {
+      const back = monoPrev.current ?? { baseFilter: 'provia' as Recipe['baseFilter'], simId: 'sim-provia', name: 'PROVIPES' };
+      setRecipe((prev) => ({ ...prev, baseFilter: back.baseFilter, name: back.name, isCustom: true }));
+      setSimId(back.simId);
+    } else {
+      monoPrev.current = { baseFilter: recipe.baseFilter, simId, name: recipe.name };
+      setRecipe((prev) => ({ ...prev, baseFilter: 'monochrome', name: 'MONOCHROME', isCustom: true }));
+      setSimId(null);
+    }
+  };
+
   // The main chip row of the active tab, exactly the phone's per-tab sets.
   const chips: ChipDef[] = (() => {
     switch (tab) {
@@ -1401,7 +1421,10 @@ export function Workspace() {
           groupChip('cxb'),
         ];
       case 'fx':
-        return paramChips(PARAM_DEFS.filters);
+        return [
+          { key: 'mono', label: 'MONOCHROME', active: monoOn, onClick: toggleMono },
+          ...paramChips(PARAM_DEFS.filters),
+        ];
       case 'frame':
         // WATERMARK is a chip like the rest: its own sub-panel opens as the next
         // column (see the wm column in the render), so the frame chips stay put.