fix(reader): correct slider side-button direction and legend on X3 (#2402) (#2428)

## Summary

* **What is the goal of this PR?** Fixes #2402. On the X3, the "Time to
Sleep"
picker's 5-minute side buttons were inverted (left increased, right
decreased)
  and the on-screen legend didn't match the physical buttons.
* **What changes are included?**
  * Flip the large-step (±5 min) direction on X3 so the left side button
decreases and the right increases, matching the layout. X4 is unchanged.
* Add a device-specific step-hint string
(`STR_SLEEP_TIMER_STEP_HINT_X3`)
selected via `gpio.deviceIsX3()`. X3 shows `+/-: 1 min Side buttons: 5
min`;
    X4 keeps the original `Left/Right: 1 min  Up/Down: 5 min`.
* Same fix applied to both slider pickers that read the raw up/down side
    buttons: the Time-to-Sleep picker and the "Go to %" picker
(`EpubReaderPercentSelectionActivity`), each with its own X3 hint string
    (`STR_SLEEP_TIMER_STEP_HINT_X3`, `STR_PERCENT_STEP_HINT_X3`).

## Additional Context

* **Root cause:** the X3's side buttons sit one on each edge of the
screen
(power on top), whereas the X4 has a vertical up/down rocker on the
right
edge. So `BTN_UP` is physically the *left* button on X3 but the *top*
button
  on X4. The picker mapped up→+5 / down→−5 unconditionally, which reads
naturally on the X4 rocker but inverted on the X3's left/right buttons.
The
  static legend ("Up/Down: 5 min") had the same X3-only mismatch.
* The codebase already detects the device at runtime and handles this
layout
difference elsewhere (e.g. `LyraTheme::drawSideButtonHints` draws "Up on
left,
Down on right" for X3), so this reuses the same `gpio.deviceIsX3()`
signal.
* Swept the codebase for other side-button sliders:
`ClockOffsetActivity` already
uses the `Next/Previous` (`ButtonNavigator`) abstraction, which resolves
to
left=decrement / right=increment on X3, so it needs no change. List/page
navigation (Wi-Fi, KOReader sync, BMP viewer) and the keyboard cursor
are also
unaffected. The two slider pickers above were the only ones using raw
up/down.
* **X4 is untouched** — same direction, same legend wording as before.
* **Tested on X3 hardware:** left side button now decreases, right
increases, and
  the legend matches. X4 not yet tested on device (no unit on hand); its
  behaviour and strings are unchanged from master.
* **Translations:** the new `STR_SLEEP_TIMER_STEP_HINT_X3` and
`STR_PERCENT_STEP_HINT_X3` were added to all 26 languages, but the
non-English
  ones are AI-generated and would benefit from a native-speaker pass —
particularly Hebrew (RTL ordering with a leading `+/-`), Kazakh, and
Vietnamese.

---

### AI Usage

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

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/activities/settings/SettingsActivity.cpp

@@ -336,10 +336,9 @@ void SettingsActivity::syncQuickResumeTimeoutForSleepScreen(bool sleepScreenChan
 void SettingsActivity::openSleepTimeoutPicker() {
   startActivityForResult(
       std::make_unique<IntervalSelectionActivity>(
-          renderer, mappedInput, "SleepTimeoutInterval", StrId::STR_TIME_TO_SLEEP, StrId::STR_SLEEP_TIMER_STEP_HINT,
-          SETTINGS.sleepTimeoutMinutes, CrossPointSettings::MIN_SLEEP_TIMEOUT_MINUTES,
-          CrossPointSettings::MAX_SLEEP_TIMEOUT_MINUTES, 1, 5, StrId::STR_SLEEP_TIMER_VALUE_FORMAT, false, true,
-          StrId::STR_SLEEP_NEVER),
+          renderer, mappedInput, "SleepTimeoutInterval", StrId::STR_TIME_TO_SLEEP, SETTINGS.sleepTimeoutMinutes,
+          CrossPointSettings::MIN_SLEEP_TIMEOUT_MINUTES, CrossPointSettings::MAX_SLEEP_TIMEOUT_MINUTES, 1, 5,
+          StrId::STR_SLEEP_TIMER_VALUE_FORMAT, false, true, StrId::STR_SLEEP_NEVER),
       [this](const ActivityResult& result) {
         if (!result.isCancelled) {
           SETTINGS.sleepTimeoutMinutes = static_cast<uint8_t>(std::get<IntervalResult>(result.data).value);