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/reader/EpubReaderPercentSelectionActivity.cpp

@@ -1,8 +1,11 @@
 #include "EpubReaderPercentSelectionActivity.h"
 
 #include <GfxRenderer.h>
+#include <HalGPIO.h>
 #include <I18n.h>
 
+#include <cstdio>
+
 #include "MappedInputManager.h"
 #include "components/UITheme.h"
 #include "fontIds.h"
@@ -51,8 +54,14 @@ void EpubReaderPercentSelectionActivity::loop() {
   buttonNavigator.onPressAndContinuous({MappedInputManager::Button::Left}, [this] { adjustPercent(-kSmallStep); });
   buttonNavigator.onPressAndContinuous({MappedInputManager::Button::Right}, [this] { adjustPercent(kSmallStep); });
 
-  buttonNavigator.onPressAndContinuous({MappedInputManager::Button::Up}, [this] { adjustPercent(kLargeStep); });
-  buttonNavigator.onPressAndContinuous({MappedInputManager::Button::Down}, [this] { adjustPercent(-kLargeStep); });
+  // On X3 the side buttons sit on the left/right edges of the screen rather than as a vertical up/down
+  // rocker (X4), so BTN_UP is physically the left button and BTN_DOWN the right one. Flip the large-step
+  // direction there so the left button decreases and the right button increases, matching the layout.
+  const int upDelta = gpio.deviceIsX3() ? -kLargeStep : kLargeStep;
+  const int downDelta = gpio.deviceIsX3() ? kLargeStep : -kLargeStep;
+  buttonNavigator.onPressAndContinuous({MappedInputManager::Button::Up}, [this, upDelta] { adjustPercent(upDelta); });
+  buttonNavigator.onPressAndContinuous({MappedInputManager::Button::Down},
+                                       [this, downDelta] { adjustPercent(downDelta); });
 }
 
 void EpubReaderPercentSelectionActivity::render(RenderLock&&) {
@@ -89,8 +98,13 @@ void EpubReaderPercentSelectionActivity::render(RenderLock&&) {
   const int knobX = barX + 2 + fillWidth - 2;
   renderer.fillRect(knobX, barY - 4, 4, barHeight + 8, true);
 
-  // Hint text for step sizes.
-  UITheme::drawCenteredText(renderer, screen, SMALL_FONT_ID, barY + 30, tr(STR_PERCENT_STEP_HINT), true);
+  // Two-line step hint built from separate label + value strings (front buttons = fine step, side
+  // buttons = coarse step), so the layout doesn't depend on a separator hidden in translated text.
+  char line[64];
+  snprintf(line, sizeof(line), "%s %d%%", I18N.get(StrId::STR_STEP_HINT_FRONT), kSmallStep);
+  UITheme::drawCenteredText(renderer, screen, SMALL_FONT_ID, barY + 30, line, true);
+  snprintf(line, sizeof(line), "%s %d%%", I18N.get(StrId::STR_STEP_HINT_SIDE), kLargeStep);
+  UITheme::drawCenteredText(renderer, screen, SMALL_FONT_ID, barY + 52, line, true);
 
   // Button hints follow the current front button layout.
   const auto labels = mappedInput.mapLabels(tr(STR_BACK), tr(STR_SELECT), "-", "+");