fix(ui): keep theme floor color on layer hover-exit

applyTheme set layer colors via repaintLayers(theme.floor) but never
updated the #floor_color input, which is the source of truth on the
picker priority path. Hover-exit repaint (repaintLayers() / Layer.getColor())
read the stale input and reverted layers to the dark default. Restore the
v2 behaviour of writing the theme floor into #floor_color.

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

Author: vagkaratzas

frontend/e2e/themes.spec.ts

@@ -30,6 +30,10 @@ test('theme buttons attach on load and switch scene theme (undoable)', async ({
 
   await page.locator('#lightThemeButton').click()
   expect(await page.evaluate(edgeColor())).toBe('#5c5c5c')
+  // Regression: floor_color input is the picker-path source of truth, so a
+  // theme toggle must update it — otherwise hover-exit repaint reverts layers
+  // to the stale (dark) color.
+  await expect(page.locator('#floor_color')).toHaveValue('#8aa185')
 
   await page.locator('#grayThemeButton').click()
   expect(await page.evaluate(edgeColor())).toBe('#6e2a5a')

frontend/src/actions/themes.ts

@@ -63,8 +63,14 @@ export function applyTheme(name: string, fromInit = false): void {
 
   if (!fromInit) {
     // v2 repaintLayersFromPicker: switch layers to picker priority with the
-    // theme's floor color
+    // theme's floor color. The floor_color input is the source of truth on the
+    // picker path (repaintLayers()/Layer.getColor() read it on hover-exit), so
+    // update it too — otherwise hover-exit reverts to the stale (dark) color.
     ctx.layerColorPrioritySource = 'picker'
+    const floorInput =
+      typeof document !== 'undefined' &&
+      (document.getElementById('floor_color') as HTMLInputElement | null)
+    if (floorInput) floorInput.value = theme.floor
     repaintLayers(theme.floor)
     redrawIntraLayerEdges()
     ctx.renderInterLayerEdgesFlag = true