feat: Seamless sleep/wake screens for displaying book pages during deep sleep (#2064)

<img width="605" height="454" alt="image"
src="https://github.com/user-attachments/assets/bfd84afe-3b58-436e-9a5d-539af3ec3d4e"
/>

Actualized "Last" sleep screen setting from previous PRs, rebranded as a
~~`Seamless Sleep`~~ `Page as Sleep Screen` option with more
improvements.


https://github.com/user-attachments/assets/59029ba6-007e-4841-abfa-f680d7e98b79

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New option: `Page as Sleep Screen` - `Never (default)`, `After Timeout`,
`Always`

When enabled, it seamlessly sleeps on timeout or power off, making a
fast refresh for the moon icon. When waking up, we still show the last
page, instead of the boot screen, making it fully seamless.

I tried different icons such as "refresh arrow" and others, but they
looked not as nice as 3 simple dots.

With this mode, the device turns off 4 seconds faster. And has 6 seconds
less delay when turning back on. Much more responsive.

Previously, even a 10-minute timeout sometimes wasn't enough, and I'd
worry about seeing the book cover. It's now easier to use a shorter
sleep timeout: if I get distracted during a reading session but don't
want to stop, the new screen is much more inviting to come back to.

---

Did you use AI tools to help write this code? _**PARTIALLY**_.

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Test v1.3.0 firmware.bin file
[download](https://github.com/user-attachments/files/28015193/firmware.zip)

Based on PRs #410 and #495

Closes #400, #1649

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Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

src/CrossPointSettings.h

@@ -155,6 +155,13 @@ class CrossPointSettings {
 
   enum TILT_PAGE_TURN { TILT_OFF = 0, TILT_NORMAL = 1, TILT_NVERTED = 2, TILT_PAGE_TURN_COUNT };
 
+  enum SEAMLESS_SLEEP_SCREEN {
+    SEAMLESS_NEVER = 0,
+    SEAMLESS_AFTER_TIMEOUT = 1,
+    SEAMLESS_ALWAYS = 2,
+    SEAMLESS_SLEEP_SCREEN_COUNT
+  };
+
   // Sleep screen settings
   uint8_t sleepScreen = DARK;
   // Sleep screen cover mode settings
@@ -242,6 +249,8 @@ class CrossPointSettings {
   uint8_t tiltPageTurn = TILT_OFF;
   // Language setting (Language enum index, default 0 = EN)
   uint8_t language = 0;
+  // Seamless sleep: keep current content visible with moon icon instead of showing sleep screen
+  uint8_t seamlessSleepScreen = SEAMLESS_NEVER;
 
   ~CrossPointSettings() = default;