fix(edit): correct the recurrence picker's read-out and end handling

Review follow-ups to the picker redesign, plus the bugs it exposed.

The live read-out now renders customResult itself — the exact string OK
would save — instead of rebuilding a parallel rule with `interval ?: 1`
and `end ?: Never` fallbacks, which let it confidently describe a rule
that differed from the selected controls whenever a field was invalid.

Shrink the invalid space behind it: a blank amount field reads as its
visible placeholder (1 / 10) rather than as an error, and backing out of
the date picker falls back to "never" instead of stranding a dateless
"on a date". Only an out-of-range 0 remains invalid, and that now says so
rather than greying out OK with no cause.

UNTIL displayed the day after the one picked for zones behind UTC:
toRRule deliberately writes the end of the chosen *local* day expressed
in UTC (the provider applies UNTIL coarsely), so the read side must
convert back before taking the date. Fixes the detail screen too, and
untilLocalDate is extracted so it can be tested.

Also: hoist a remember() out of a conditional (a slot that appears and
disappears breaks positional memoisation), match GroupedSurface's 22dp
corners instead of a drifted local 20dp copy, move the cards onto
GroupedSurface, drop the segmented row's icon slot so longer unit labels
fit, and reserve two lines so the stack stops shifting as the phrase
grows with each weekday.

Extract SettingsPrefs.firstDayOfWeek(scope), replacing three copies that
had drifted onto different initialValues.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/res/values/strings.xml

@@ -149,6 +149,9 @@
     <string name="recurrence_unit_weeks">weeks</string>
     <string name="recurrence_unit_months">months</string>
     <string name="recurrence_unit_years">years</string>
+    <!-- Shown in place of the live rule read-out when an amount field holds a
+         value outside 1–999 (i.e. 0), so the greyed-out OK button has a reason. -->
+    <string name="event_edit_recurrence_incomplete">Enter a number from 1 to 999</string>
     <string name="event_edit_recurrence_ends">Ends</string>
     <string name="event_edit_recurrence_end_never">Never</string>
     <string name="event_edit_recurrence_end_until">On a date</string>