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

web/src/api/client.ts

@@ -147,6 +147,10 @@ export const api = {
     req<void>(`/suggestions/${id}/accept`, { method: 'POST' }),
   dismissSuggestion: (id: string) =>
     req<void>(`/suggestions/${id}/dismiss`, { method: 'POST' }),
+  // Simplified-Chinese rendering of a suggestion's explanation, for the Ask Petal
+  // opening bubble (the explanation itself stays English in the card body).
+  translateSuggestion: (id: string) =>
+    req<{ translation: string }>(`/suggestions/${id}/translate`, { method: 'POST' }),
 
   // Version history. listVersions returns metadata only (no bodies); getVersion
   // loads one full snapshot for preview; snapshotDoc takes an explicit restore