Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@src/components/LyricsDisplay.test.tsx`:
- Around line 1-57: Add tests in LyricsDisplay.test.tsx to cover the checkbox
and info-button interactions: render LyricsDisplay (wrapped with
QueryClientProvider if needed), pass vi.fn() spies for onLineCheckboxClick and
onLineInfoClick props, use testing-library fireEvent or userEvent to click the
checkbox for a specific lyric and the info button for that lyric, and assert the
corresponding spy was called with the expected lyric id or payload; locate
handlers by prop names onLineCheckboxClick and onLineInfoClick and target
elements by role/label/text matching the line content used in the mocked data.

In `@src/components/LyricsDisplay.tsx`:
- Line 57: Replace the hardcoded "persian" fallback used when computing RTL with
the new renamed filter value so unknown source languages default to LTR: update
the isOriginalRTL assignment (the variable isOriginalRTL that calls
isRTLLanguage(sourceLanguage || "persian")) to use "original" (or otherwise pass
an empty/undefined value) instead of "persian" so the RTL check aligns with the
rename and treats unknown languages as LTR.

In `@src/routes/song`.$songId.tsx:
- Around line 822-844: The handleToggleWordLearned callback calls
toggleWordLearnedMutation but never handles failures; update
handleToggleWordLearned to add a .catch on the toggleWordLearnedMutation promise
(or use try/catch if converting to async) that logs the error (e.g.,
console.error or processLogger), shows user-facing feedback (toast or set an
error state) and, if you applied any optimistic UI changes, revert them;
reference the toggleWordLearnedMutation call inside handleToggleWordLearned and
also keep logPracticeMutation and toggleWordLearnedFn behavior unchanged for the
success path.
- Around line 506-511: Refactor duplicated seek-guard blocks to use the existing
seekTo helper: replace the inline pattern in handlePlaybackModeChange,
handleLyricLineClick, and handleWordModalClose with calls to
seekTo(sortedLyrics[lineIndexToPlay].startTime) (or the appropriate startTime
for the clicked/closed line) and remove the manual isSeekingRef/current and
setTimeout logic; to fix handlePlaybackModeChange you must move the seekTo
function definition above handlePlaybackModeChange (or hoist it) and then add
seekTo to handlePlaybackModeChange’s dependency array so hooks remain correct;
keep references to playerRef, isSeekingRef, sortedLyrics and lineIndexToPlay as
before.

---

Outside diff comments:
In `@src/components/LyricsDisplay.tsx`:
- Line 83: The ref array lineRefs is incorrectly typed as (HTMLButtonElement |
null)[] while it stores div elements; update the type used in the useRef
declaration (lineRefs) to (HTMLElement | null)[] (or a more specific
HTMLDivElement | null[]) and adjust any casts (the setter that currently uses el
as HTMLButtonElement | null) to use the matching HTMLElement/HTMLDivElement type
so calls like scrollIntoView remain correctly typed; locate useRef(...) for
lineRefs and the ref assignment where el is cast and change both to the
consistent HTMLElement/HTMLDivElement type.

In `@src/hooks/useAnonymousProgress.ts`:
- Around line 12-19: The WordProgressItem interface in useAnonymousProgress.ts
uses a Persian-specific field name (WordProgressItem.persian); rename this field
to a neutral name like original across the type and all usages
(serialization/deserialization, reads/writes, and any accessors in
useAnonymousProgress) and add a simple migration when loading saved progress
from localStorage to map old persian keys to original (similar to the
languageFilter migration) so existing data is preserved; update all code that
reads/writes WordProgressItem (including any JSON.parse/JSON.stringify paths) to
handle both old and new property names until migration is complete.

In `@src/routes/song`.$songId.tsx:
- Around line 382-409: The effect currently calls logPracticeMutation({
eventType: "audio_time", value: 1 }) every second for authenticated users which
will cause high write volume; change useEffect logic (the effect using
lastActivityRef, IDLE_THRESHOLD_MS, shouldCountTime, practiceSecondsRef,
isAuthenticated, logPracticeMutation, logPracticeFn) to accumulate seconds in a
local counter (e.g., practiceBatchRef) and only call logPracticeMutation with
the batched value on a longer flush interval (e.g., 10–30s) or when user becomes
idle/unmounts, while keeping logPracticeFn behavior for anonymous users; ensure
you flush remaining batched seconds on cleanup/unload and reset counters
appropriately so real-time header updates remain reasonably fresh but mutations
are throttled.