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/util/IntervalSelectionActivity.h

@@ -11,13 +11,12 @@ class GfxRenderer;
 class IntervalSelectionActivity final : public Activity {
  public:
   explicit IntervalSelectionActivity(GfxRenderer& renderer, MappedInputManager& mappedInput, const char* activityName,
-                                     StrId titleId, StrId stepHintId, int initialValue, int minValue, int maxValue,
-                                     int smallStep, int largeStep, StrId valueFormatId = StrId::STR_NONE_OPT,
+                                     StrId titleId, int initialValue, int minValue, int maxValue, int smallStep,
+                                     int largeStep, StrId valueFormatId = StrId::STR_NONE_OPT,
                                      bool readerActivity = false, bool ignoreInitialConfirmRelease = false,
                                      StrId maxBoundaryLabelId = StrId::STR_NONE_OPT)
       : Activity(activityName, renderer, mappedInput),
         titleId(titleId),
-        stepHintId(stepHintId),
         valueFormatId(valueFormatId),
         maxBoundaryLabelId(maxBoundaryLabelId),
         value(initialValue),
@@ -35,7 +34,6 @@ class IntervalSelectionActivity final : public Activity {
 
  private:
   StrId titleId;
-  StrId stepHintId;
   StrId valueFormatId;
   StrId maxBoundaryLabelId;
   int value;
@@ -49,4 +47,5 @@ class IntervalSelectionActivity final : public Activity {
 
   void adjustValue(int delta);
   int clampedValue(int candidate) const;
+  void drawStepHintLine(int y, StrId labelId, int step);
 };