Add configurable gap between pages option for webtoon reading

[madeindjs]

Add a new 'Gap between pages' appearance setting (PAGE_GAP) that controls the spacing between consecutive pages in continuous/vertical mode. Previously this was a hardcoded 2px constant. Setting it to 0 gives seamless webtoon reading where a single image spans multiple pages without visible separators.

Also fix the debug-only dump() assertion that crashed when scrolling with pageGap=0: sub-pixel float rounding during scroll could make adjacent pages overlap by a fraction of a pixel across their full width, triggering the assertion. Now requires both width and height overlap to exceed 1px before flagging an intersection.

Screenshots

before	after

[max-kammerer]

@madeindjs Thank you for contribution!

1. You use the legacy mechanism for property updates — there is a newer one: pref(...) returns a Preference<T> : LiveData<T> (prefs/Preference.kt:20), with DRAW_PAGE_BORDER as a neighbouring example (GlobalOptions.kt:244) and subscribers using .observe(...) (view/OrionStatusBar.kt:39, OrionViewerActivity.kt:110).

   Two consequences of the current approach: the new value is only applied to pages that are (re)added afterwards, so already laid out pages keep the old gap until they scroll off screen and are recreated; and globalOptions.pageGap is read from SharedPreferences in the draw hot path (PageView.kt:279), which does a getString() plus a parse per page per frame. Preference/LiveData caches the value and notifies observers, which solves both.

2. I think it's worth looking for another approach instead of the 1px overlap and the relaxed assertion. It could stay as a temporary workaround in the meantime.

3. There is a dedicated checkbox for border drawing, and I'd prefer not to mix that logic with the gap.

4. You use pixels — dp would probably be a better unit here.

5. Two attributes in user_pref_appearance.xml have no effect and can be dropped: app:showSeekBarValue belongs to androidx SeekBarPreference and does nothing on OrionEditTextPreference, and android:inputType isn't read by EditTextPreference — the input type is already set in OrionEditPreference.kt:100. If you did want a slider, there is prefs/SeekBarPreferenceAsText.kt, which persists an int as a string exactly as needed here.

[max-kammerer]

@madeindjs please also use https://github.com/max-kammerer/orion-viewer/blob/master/docs/architecture.md. Feel free to ask any questions