Re-run OCR immediately when the OCR source changes

[bee-san]

Follow-up to #81.

Problem

Changing the OCR source from the long-press menu on the reader OCR action didn't visibly do anything. setOcrSource() stored the preference, cancelled the in-flight scan and cleared ocrScannedChapterIds, but nothing started a new scan — the chapter-wide OCR pass only restarts from onPageSelected -> scanOcrPages(). So picking Google Lens while sitting on a page left the reader idle until the user swiped.

Fix

setOcrSource() now restarts the scan for the currently selected page right away, when the OCR overlay is enabled:

if (isOcrEnabled()) {
    getSelectedReaderPage()?.let { scanOcrPages(it) }
}

This mirrors what the existing ocrOverlayEnabled().changes() collector already does when OCR is toggled on.

Note the visible page was already being re-OCR'd by the setOcrEnabled(false) / setOcrEnabled(true) dance in ReaderActivity.selectOcrSourceFromReader. What was missing is the chapter-wide background rescan under the new source (and its progress HUD), which is what this restores. No duplicate network work: both paths go through getOcrBlocks(), which dedupes concurrent fetches via ocrInFlight keyed on (chapterId, pageIndex, ocrSource).

Drive-by

ReaderOcrSourceTest.availableSources() was still calling the old single-argument signature and doesn't compile on main — #81 added the mokuroAvailable parameter to ReaderOcrSource.availableSources() but didn't update the test. One call site, two arguments. Without this testReleaseUnitTest fails to compile, so it's included here rather than left broken.

Verification

• ./gradlew testReleaseUnitTest — passes (fails to compile on main without the test fix above)