Add CANCEL to the watermark editor, scope the mark's touch layer to that editor, and stop a library frame following the user onto the camera

The custom watermark captured EVERY image touch whenever the chip was ON
with text, in both modes: after switching from a library photo to the
camera there was no way to give the image back to tap-to-focus, and a
frame picked for the still rode along onto the live preview (and into the
capture).

- AdjustPanel: CANCEL chip next to ACCEPT. It drops the un-accepted draft
  and restores the last ACCEPTED mark (none if nothing was accepted).
- Viewfinder: draws the mark whenever the chip is ON with text, but only
  lets it CAPTURE touches while its editor is open (new wmEditing prop).
  A double-tap ON the mark still re-opens that editor from the library
  viewer.
- App: ACCEPT and CANCEL now close the editor (that closes the placement
  layer with it). Switching mode closes the deck panels, and leaves the
  frame behind: camera mode has no FRAME picker to undo one.

Verified on device (9a6a7277): camera tap locks AE/AF with the watermark
left ON; library polaroid no longer appears on the live preview after the
switch; CANCEL reverts the chip to CUSTOM WATERMARK OFF and closes the
panel; ACCEPT closes it and the next capture embeds the mark (amber text
found at 0.5/0.375 of the 3000x4000 export), while a capture before
ACCEPT carries none.

src/components/AdjustmentPanel.tsx

@@ -58,6 +58,10 @@ interface AdjustmentPanelProps {
   // ACCEPT: copy the live draft into the mark the export burns in. Edits stay a
   // draft (viewfinder preview only) until this is pressed.
   onAcceptCustomWm: () => void;
+  // CANCEL: throw the un-accepted draft away — back to the mark that was last
+  // ACCEPTED (or none at all) — and close the editor. The way out of a running
+  // watermark edit that previously did not exist.
+  onCancelCustomWm: () => void;
   // Bumped by a double-tap on the mark: put the caret back in the text input.
   wmEditNonce: number;
 }
@@ -139,6 +143,7 @@ export default function AdjustmentPanel({
   customWm,
   onUpdateCustomWm,
   onAcceptCustomWm,
+  onCancelCustomWm,
   wmEditNonce,
   onToggleRotation,
 }: AdjustmentPanelProps) {
@@ -526,6 +531,13 @@ export default function AdjustmentPanel({
                 active: true,
                 onPress: onAcceptCustomWm,
               })}
+            {customWm.enabled &&
+              renderChip({
+                key: 'custom-wm-cancel',
+                label: 'CANCEL',
+                active: false,
+                onPress: onCancelCustomWm,
+              })}
             {customWm.enabled && renderChip(groupChip('wmColor'))}
             {customWm.enabled && renderChip(groupChip('wmSize'))}
             {customWm.enabled && renderChip(groupChip('wmFont'))}