Suggestions: right-margin comment rail + Mandarin explanations

Surface every outstanding suggestion as a card in the right-hand
whitespace, vertically aligned to the text it flags — so the writer sees
the whole queue at once instead of hovering each highlight. Cards stack
with collision avoidance, link both ways with their highlight (hover/click
↔ soft text wash, driven through the decoration plugin so it survives
edit repaints), and carry the same Accept / Dismiss / Ask Petal actions.
The rail is a progressive enhancement: it mounts only when there's room
beside the editor, otherwise the existing inline hover card is unchanged.
Stacked cards that reach the bottom-right corner tuck behind the
companion mascot (z-order).

When a card is expanded, the Ask Petal bubble now opens with the
Simplified-Chinese translation of the explanation (the English stays in
the card body) instead of repeating the same text twice — a new
POST /api/suggestions/{id}/translate one-shot LLM endpoint, loaded
lazily on open with an English fallback.

Verified live against the local LLM via the uitest harness: rail
stacking, hover↔text wash, expand/Ask Petal, accept-from-rail, narrow
fallback, and the Mandarin bubble.

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

internal/llm/prompts.go

@@ -165,3 +165,24 @@ func RewriteMessages(text, style string) []Message {
 		{Role: "user", Content: text},
 	}
 }
+
+// translateSystemPrompt drives the explanation translator: it renders a
+// suggestion's English explanation into Simplified Chinese so an ESL reader sees
+// the "why" in her first language. Strict about returning ONLY the translation
+// (no quotes, no pinyin, no English echo) so it can drop straight into the chat
+// bubble. Kept warm and plain — these are short, friendly one-liners.
+const translateSystemPrompt = `You are Petal, a warm writing assistant. Translate the English text the user ` +
+	`sends into natural, friendly Simplified Chinese (Mandarin). It is a short explanation of a writing ` +
+	`suggestion, written for a native Chinese speaker learning English.
+
+Respond with ONLY the Simplified Chinese translation. No quotation marks, no pinyin, no English, no preamble — ` +
+	`just the translated sentence.`
+
+// TranslateMessages builds the message array for translating one short English
+// explanation into Simplified Chinese.
+func TranslateMessages(text string) []Message {
+	return []Message{
+		{Role: "system", Content: translateSystemPrompt},
+		{Role: "user", Content: text},
+	}
+}

internal/llm/translate.go

@@ -0,0 +1,23 @@
+package llm
+
+import (
+	"context"
+)
+
+// RunTranslate renders a short English explanation into Simplified Chinese. It
+// is a one-shot Complete (the result seeds the Ask Petal bubble), kept at a low
+// temperature so the translation is faithful rather than creative. Output is
+// trimmed of any stray surrounding quotes the model may add.
+func RunTranslate(ctx context.Context, client LLMClient, text string) (string, error) {
+	out, err := client.Complete(ctx, CompletionRequest{
+		Messages:          TranslateMessages(text),
+		MaxTokens:         512,
+		Temperature:       0.2,
+		TopP:              0.9,
+		RepetitionPenalty: 1.1,
+	})
+	if err != nil {
+		return "", err
+	}
+	return cleanRewrite(out), nil
+}