Drop the daytime widget, use clock everywhere

Clock and daytime were near-duplicates with different click semantics
(clock toggles between formats on left-click and opens the timezone
selector on right-click; daytime was non-pressable with a hover tooltip).
Standardize on clock, since it's the more capable of the two.

- Delete widgets/daytime.qml and its FirstPartyWidgets entry.
- Migrate shell-defaults.json, shell.qml's builtin fallback, Bar.qml's
  fallback bar config, and the settings panel's builtin defaults so the
  center anchor and center-section entry both reference "clock".
- Rename the daytimeSettings form to clockSettings and rephrase
  "Tooltip format" → "Alternate format (click to toggle)" so the field
  label matches clock's actual behavior.
- Update the gallery dropdown options and bar/README docs.

shell/shell-defaults.json

@@ -7,7 +7,7 @@
   "bar": {
     "position": "top",
     "transparent": false,
-    "centerAnchor": "daytime",
+    "centerAnchor": "clock",
     "layout": {
       "left": [
         {
@@ -19,7 +19,7 @@
       ],
       "center": [
         {
-          "id": "daytime",
+          "id": "clock",
           "format": "dddd HH:mm",
           "formatAlt": "dd MMMM yyyy",
           "verticalFormat": "HH\n\u2014\nmm"