fix: a NUL byte was hiding two source files from grep, git and diff

TaskHost.tsx used a literal NUL as the delimiter in its drag-and-drop
payload, and tools/dict/build.mjs used one to join headword and part of
speech. Both work at runtime. Both also make the file *binary* to every
text tool: git shows "Bin 12259 bytes" instead of a diff, and grep prints
nothing at all for a match.

That is not hypothetical. Searching TaskHost.tsx for "<input" came back
empty three times while reviewing it, which is how its four exercise
inputs came to be reported as absent -- and why the accessibility defect
in them went unseen. Written as the escape \u0000 the value is identical
and the file stays text.

test/source-hygiene.test.ts fails on any control byte in a source file, so
this cannot come back quietly.

With the files readable again, the sweep the NUL had been hiding: eleven
form controls had no accessible name. The exercise blanks announced only
an ellipsis, and the part-of-speech select announced nothing. A
placeholder is not a label -- it disappears the moment you type. All
eleven now carry one, named after the thing they answer.

`npm run lint` gains --max-warnings 0. exhaustive-deps is configured as a
warning, so a hooks-dependency bug would have passed CI silently.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/src/ui/tabs/GrammarTab.tsx

@@ -152,6 +152,7 @@ function ConjugationTrainer() {
             className="ko"
             value={value}
             placeholder="…"
+            aria-label={`Conjugate ${question.dict} — ${mode === "past" ? "past 반말" : "반말"}`}
             onChange={(e) => {
               composer.onExternalInput();
               setValue(e.target.value);

app/src/ui/tabs/VocabTab.tsx

@@ -108,6 +108,7 @@ export function VocabTab() {
               type="search"
               value={query}
               placeholder="Search 한글 or English…"
+              aria-label="Search your words in 한글 or English"
               onChange={(e) => setQuery(e.target.value)}
             />
             <button className="btn" onClick={() => setAdding((a) => !a)} aria-expanded={adding}>
@@ -142,16 +143,19 @@ export function VocabTab() {
                 className="ko"
                 value={draft.headword}
                 placeholder="한글"
+                aria-label="The word, in 한글"
                 required
                 onChange={(e) => setDraft({ ...draft, headword: e.target.value })}
               />
               <input
                 value={draft.gloss}
                 placeholder="What it means"
+                aria-label="What it means"
                 required
                 onChange={(e) => setDraft({ ...draft, gloss: e.target.value })}
               />
               <select
+                aria-label="Part of speech"
                 value={draft.pos}
                 onChange={(e) => setDraft({ ...draft, pos: e.target.value })}
               >

app/src/ui/tutor/WordRail.tsx

@@ -146,6 +146,7 @@ export function WordRail({ words, revealed, onReveal }: WordRailProps) {
           type="search"
           value={query}
           placeholder="Look a word up…"
+          aria-label="Look a word up"
           onChange={(e) => setQuery(e.target.value)}
         />
       </div>