Add a web UI to the clock firmware

- Web UI on port 80: live view of the display, choose the page (clock or
  a bitmap), bitmaps (list, show, live, edit, delete), draw / image upload
  with conversion in the browser, live mode, settings, Home Assistant
- Live view from a shadow buffer in the BROSE9323 driver (shownPixel)
- Settings in NVS with the original behaviour as defaults: schedule times,
  quarter hours, yawning Pikachu, No WiFi screen, flip time, status LED,
  Home Assistant broker (restart on change)
- Bitmaps on LittleFS; new 320 KB partition, ota slots now 1.31 MB
- DisplayController: pages, live mode, settings; app.cpp holds the objects
- README for the recovery app; docs for the web UI
- Tests: settings, time parsing, bitmap format and names, schedule with
  settings, pages and live mode (95 total)

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

src/esp32/logic/settings.h

@@ -0,0 +1,59 @@
+#pragma once
+
+#include <stdint.h>
+
+// Everything that can be changed in the web UI. Stored in NVS (system/settings_store),
+// the defaults are the original behaviour. Pure logic, tested in test/test_esp32.
+//
+// Times are minutes since midnight (8:00 = 480).
+
+struct ScheduleSettings
+{
+	bool enabled = true;            // false: the display is always on
+	uint16_t day_start = 8 * 60;    // display on from
+	uint16_t late_start = 22 * 60;  // from here only at the quarter hours (see quarter_hours)
+	bool quarter_hours = true;      // false: on all the time until night_start
+	uint16_t night_start = 23 * 60 + 31; // display off from (up to 24:00 = never)
+};
+
+struct ClockSettings
+{
+	bool evening_enabled = true;    // yawning Pikachu in the evening
+	uint16_t evening_start = 17 * 60;
+	bool no_wifi_screen = true;     // "No WiFi" instead of the clock when WiFi is down
+};
+
+struct HomeAssistantSettings
+{
+	bool enabled = true;
+	char host[64] = "";             // empty: BROKER_ADDR from secrets.h
+	uint16_t port = 1883;
+	char user[32] = "";
+	char password[64] = "";
+};
+
+// What is shown during the day: "clock" or the name of a bitmap
+const int PAGE_NAME_SIZE = 25;
+#define PAGE_CLOCK "clock"
+
+struct Settings
+{
+	char page[PAGE_NAME_SIZE] = PAGE_CLOCK;
+	ScheduleSettings schedule;
+	ClockSettings clock;
+	HomeAssistantSettings home_assistant;
+	uint16_t flip_time = 225; // flip pulse length in us, sent to the Nano
+	bool status_led = true;
+};
+
+const uint16_t MINUTES_PER_DAY = 24 * 60;
+const uint16_t SETTINGS_FLIP_TIME_MIN = 50;
+const uint16_t SETTINGS_FLIP_TIME_MAX = 2000;
+
+// Checks the values. Returns nullptr when they are fine, otherwise a message for the user.
+const char *validateSettings(const Settings &settings);
+
+// "HH:MM" (00:00 - 24:00) <-> minutes since midnight
+bool parseTime(const char *text, uint16_t &minutes);
+// out must hold 6 characters
+void formatTime(uint16_t minutes, char *out);