Fix forecast showing yesterday's days for a while after midnight

Weather's daily forecast starts with "today" as of when check() last
ran -- correct at fetch time, but that cached state sits until the
next poll (up to update_minutes later, 30 min by default). Right
after local midnight, that means the forecast keeps showing the old
day first (e.g. Fr Sa So Mo Di at 00:30 Saturday) until the next
scheduled poll happens to land after the rollover.

ScreenCache now also tracks the calendar date it was last checked on
and treats itself as stale on a date change, independent of
poll_interval -- so the very next tick after midnight re-checks
weather (and any other date-sensitive screen) immediately instead of
waiting out the rest of the interval. Verified with a unit test
simulating a cache populated "yesterday".

scheduler.py

@@ -15,6 +15,7 @@ import hashlib
 import logging
 import threading
 import time
+from datetime import date
 from pathlib import Path
 
 from PIL import Image
@@ -32,8 +33,17 @@ class ScreenCache:
     def __init__(self):
         self.state = None
         self.checked_at: float = 0.0
+        self.checked_date: date | None = None
 
     def is_stale(self, poll_interval: int) -> bool:
+        # Also stale across a local-midnight rollover, independent of
+        # poll_interval -- otherwise a screen whose data is inherently
+        # date-bound (weather's daily forecast labels, its "today" hourly
+        # graph) keeps showing yesterday's data for up to poll_interval
+        # after midnight, since the cached state was genuinely correct
+        # when fetched, just not anymore now that the date has turned over.
+        if self.checked_date != date.today():
+            return True
         return (time.time() - self.checked_at) >= poll_interval
 
 
@@ -197,6 +207,7 @@ class Scheduler:
         if cache.is_stale(screen.get_poll_interval(settings)):
             cache.state = self._check(screen_id)
             cache.checked_at = time.time()
+            cache.checked_date = date.today()
             checked = True
         return screen_id, cache.state, checked