Code-review follow-ups: httputil, validation caps, a11y

Backend:
- Extract shared internal/httputil (WriteJSON/ErrorJSON/BadRequest/
  ServerError); drop the triple-duplicated helpers in docs, suggestions,
  vocab. ServerError now logs the real error and returns a generic 500 so
  raw DB/internal errors never reach the client.
- vocab capture: validate doc_id ownership (blank -> none, unknown -> 400
  instead of a leaked FK 500); rune-safe clamp word/gloss/definition/
  phonetic/example.
- vocab review(): wrap the read-modify-write in a transaction (TOCTOU).
- /api request-size cap via MaxBytesReader middleware (2 MiB), exempting
  /api/images (own 10 MiB limit).

Frontend:
- StatusBar: drive the checking/voicing/collocating indicators from one
  array; llmDown uses !anyBusy.
- Slide-overs: new useFocusTrap hook (focus-in, Tab trap, focus-restore)
  on GardenPanel + HistoryPanel, both role=dialog/aria-modal/aria-label.
- speech.ts: export stopSpeech(); GardenPanel cancels audio on unmount.

Tests: add doc_id-validation and field-clamp coverage; full suite green.

Claude-Session: https://claude.ai/code/session_016Yr6jELuRc7hyzYLccQKZd

internal/suggestions/rewrite.go

@@ -10,6 +10,7 @@ import (
 	"github.com/go-chi/chi/v5"
 
 	"gitea.parodia.dev/drwily/petal/internal/db"
+	"gitea.parodia.dev/drwily/petal/internal/httputil"
 	"gitea.parodia.dev/drwily/petal/internal/llm"
 )
 
@@ -36,17 +37,17 @@ func (h *Handler) rewrite(w http.ResponseWriter, r *http.Request) {
 
 	var body rewriteRequest
 	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
-		errorJSON(w, http.StatusBadRequest, "invalid request body")
+		httputil.ErrorJSON(w, http.StatusBadRequest, "invalid request body")
 		return
 	}
 
 	text := strings.TrimSpace(body.Text)
 	if text == "" {
-		errorJSON(w, http.StatusBadRequest, "no text to rewrite")
+		httputil.ErrorJSON(w, http.StatusBadRequest, "no text to rewrite")
 		return
 	}
 	if len([]rune(text)) > llm.RewriteMaxRunes {
-		errorJSON(w, http.StatusBadRequest, "selection too long to rewrite")
+		httputil.ErrorJSON(w, http.StatusBadRequest, "selection too long to rewrite")
 		return
 	}
 
@@ -58,19 +59,19 @@ func (h *Handler) rewrite(w http.ResponseWriter, r *http.Request) {
 		docID, db.LocalUserID,
 	).Scan(&exists)
 	if errors.Is(err, sql.ErrNoRows) {
-		errorJSON(w, http.StatusNotFound, "document not found")
+		httputil.ErrorJSON(w, http.StatusNotFound, "document not found")
 		return
 	}
 	if err != nil {
-		serverError(w, err)
+		httputil.ServerError(w, err)
 		return
 	}
 
 	out, err := llm.RunRewrite(r.Context(), h.Client, text, body.Style)
 	if err != nil {
-		errorJSON(w, http.StatusBadGateway, "rewrite failed: "+err.Error())
+		httputil.ErrorJSON(w, http.StatusBadGateway, "rewrite failed: "+err.Error())
 		return
 	}
 
-	writeJSON(w, http.StatusOK, rewriteResponse{Rewrite: out})
+	httputil.WriteJSON(w, http.StatusOK, rewriteResponse{Rewrite: out})
 }