Add a page schedule with rotation, Home Assistant trigger rules and a Stopwatch tab

- Schedule: daily blocks from a start time, each with up to 6 rotating pages
  or bitmaps (own duration), switching with the effect or instantly. A manual
  pick (Show tab, HA Mode) lasts until the next block; Back to schedule ends it.
- Rules: an HA state (=, is not, above, below) shows a page while true, adds it
  to the rotation while true, or shows it for N seconds when it becomes true.
  Each rule is polled at its own interval, with a 60 s back-off on errors.
- PageSelector (pure logic, tested) decides the shown page; schedule stored in
  NVS; HA sensor Shown page and button Back to schedule.
- Web UI: Schedule and Stopwatch tabs, bitmaps used by pages or rules in their
  own subsection, schedule in export/import. Version 2.6.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/esp32/net/home_assistant.h

@@ -14,6 +14,8 @@
 //   FlipDot Display Recovery  button: restart into the recovery app (for 10 minutes)
 //   FlipDot Display Stopwatch switch: the stopwatch of the dynamic pages runs
 //   FlipDot Display Stopwatch reset  button: back to 0
+//   FlipDot Display Shown page        sensor: the page or bitmap on the display now
+//   FlipDot Display Back to schedule  button: ends a manual pick (the Mode dropdown)
 //
 // The broker comes from the settings (web UI); an empty host means BROKER_ADDR, MQTT_USER
 // and MQTT_PASSWORD from secrets.h. Changes need a restart.