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/components/Editor/SuggestionHighlight.ts

@@ -16,9 +16,17 @@ export const suggestionPluginKey = new PluginKey<PluginState>('petalSuggestions'
 
 interface PluginState {
   suggestions: Suggestion[]
+  // The suggestion currently emphasized from its margin card / a hover, or null.
+  // Carried in plugin state (not an imperative DOM class) so it survives the
+  // decoration repaints that fire on every document change.
+  activeId: string | null
   decorations: DecorationSet
 }
 
+// Meta carried on a transaction to update the plugin: either a fresh suggestion
+// list or a change to which suggestion is emphasized.
+type SuggestionMeta = { suggestions: Suggestion[] } | { activeId: string | null }
+
 // mapOffset converts a character offset within a textblock's flattened text into
 // an absolute ProseMirror position, accounting for inline atoms (e.g. hard
 // breaks) that occupy a position but contribute no text.
@@ -130,14 +138,15 @@ export function findRange(doc: PMNode, search: string): { from: number; to: numb
   return result
 }
 
-function buildDecorations(doc: PMNode, suggestions: Suggestion[]): DecorationSet {
+function buildDecorations(doc: PMNode, suggestions: Suggestion[], activeId: string | null): DecorationSet {
   const decos: Decoration[] = []
   for (const s of suggestions) {
     const range = findRange(doc, s.original)
     if (!range) continue
+    const active = s.id === activeId ? ' petal-suggestion-active' : ''
     decos.push(
       Decoration.inline(range.from, range.to, {
-        class: `petal-suggestion petal-suggestion-${s.type}`,
+        class: `petal-suggestion petal-suggestion-${s.type}${active}`,
         'data-suggestion-id': s.id,
       }),
     )
@@ -148,10 +157,22 @@ function buildDecorations(doc: PMNode, suggestions: Suggestion[]): DecorationSet
 // setSuggestions pushes a new suggestion list into the plugin. Decorations are
 // rebuilt against the current document immediately.
 export function setSuggestions(state: EditorState, dispatch: (tr: Transaction) => void, suggestions: Suggestion[]) {
-  const tr = state.tr.setMeta(suggestionPluginKey, suggestions)
+  const tr = state.tr.setMeta(suggestionPluginKey, { suggestions } satisfies SuggestionMeta)
   dispatch(tr)
 }
 
+// setActiveSuggestion marks one suggestion (or none) as emphasized, rebuilding
+// the decorations so the flagged text gets the active wash. Driven by the margin
+// rail's hover/selection so the card↔text link reads both ways.
+export function setActiveSuggestion(
+  state: EditorState,
+  dispatch: (tr: Transaction) => void,
+  activeId: string | null,
+) {
+  if (suggestionPluginKey.getState(state)?.activeId === activeId) return
+  dispatch(state.tr.setMeta(suggestionPluginKey, { activeId } satisfies SuggestionMeta))
+}
+
 export const SuggestionHighlight = Extension.create({
   name: 'suggestionHighlight',
 
@@ -160,17 +181,29 @@ export const SuggestionHighlight = Extension.create({
       new Plugin<PluginState>({
         key: suggestionPluginKey,
         state: {
-          init: () => ({ suggestions: [], decorations: DecorationSet.empty }),
+          init: () => ({ suggestions: [], activeId: null, decorations: DecorationSet.empty }),
           apply(tr, value, _oldState, newState) {
-            const meta = tr.getMeta(suggestionPluginKey) as Suggestion[] | undefined
-            if (meta) {
-              return { suggestions: meta, decorations: buildDecorations(newState.doc, meta) }
+            const meta = tr.getMeta(suggestionPluginKey) as SuggestionMeta | undefined
+            if (meta && 'suggestions' in meta) {
+              return {
+                suggestions: meta.suggestions,
+                activeId: value.activeId,
+                decorations: buildDecorations(newState.doc, meta.suggestions, value.activeId),
+              }
+            }
+            if (meta && 'activeId' in meta) {
+              return {
+                suggestions: value.suggestions,
+                activeId: meta.activeId,
+                decorations: buildDecorations(newState.doc, value.suggestions, meta.activeId),
+              }
             }
             // On any document change, re-anchor by string against the new doc.
             if (tr.docChanged) {
               return {
                 suggestions: value.suggestions,
-                decorations: buildDecorations(newState.doc, value.suggestions),
+                activeId: value.activeId,
+                decorations: buildDecorations(newState.doc, value.suggestions, value.activeId),
               }
             }
             return value