feat(diffs): port the copy-context hint and Linux primary selection to Pierre

Completes the contextual-copy port. Monaco drew the "Copy context" hint as a
content widget; Pierre has no widget layer, so it becomes an overlay pinned to
the selection's client rect, shown only for multi-line selections (single-line
copies plainly, matching formatCopiedSelectionWithContext).

Primary selection (X11 middle-click paste) is restored from the shadow-root
selection, gated on isPrimarySelectionEnabled and PRIMARY_SELECTION_MAX_LENGTH
and debounced 200ms so a drag does not churn the buffer.

src/renderer/src/components/editor/pierre-diff/pierre-diff-context-copy.ts

@@ -1,5 +1,14 @@
 import { formatCopiedSelectionWithContext } from '../selection-copy'
 import { editorShortcutMatches } from '../editor-shortcuts'
+import { formatShortcutLabel } from '@/hooks/useShortcutLabel'
+import { useAppStore } from '@/store'
+import {
+  PRIMARY_SELECTION_MAX_LENGTH,
+  isPrimarySelectionEnabled,
+  setPrimarySelectionText
+} from '@/lib/primary-selection'
+
+const PRIMARY_SELECTION_DEBOUNCE_MS = 200
 
 /** Resolves the 1-based line of the row containing a selection boundary node. */
 function lineOf(node: Node | null): number | null {
@@ -19,6 +28,69 @@ export function installPierreContextualCopy(
   container: HTMLElement,
   getFileInfo: () => { relativePath: string; language: string }
 ): () => void {
+  // Why: Monaco drew this as a content widget; Pierre has no widget layer, so it
+  // is an overlay pinned to the selection's client rect instead.
+  const hint = document.createElement('div')
+  hint.className =
+    'pointer-events-none fixed z-50 rounded-md border border-border/90 bg-background px-2.5 py-1 text-xs font-medium text-foreground shadow-[0_6px_18px_rgba(15,23,42,0.18)] backdrop-blur whitespace-nowrap'
+  hint.style.display = 'none'
+  document.body.appendChild(hint)
+  let primarySelectionTimer: number | null = null
+
+  const readSelection = (): Selection | null => {
+    const root = container.querySelector('diffs-container')?.shadowRoot
+    return (root as unknown as { getSelection?: () => Selection | null })?.getSelection?.() ?? null
+  }
+
+  const hideHint = (): void => {
+    hint.style.display = 'none'
+  }
+
+  const updateHint = (): void => {
+    const selection = readSelection()
+    const text = selection?.toString() ?? ''
+    const startLine = lineOf(selection?.anchorNode ?? null)
+    const endLine = lineOf(selection?.focusNode ?? null)
+    // Why: copy-with-context is a multi-line affordance; a single line copies plainly.
+    if (!text || startLine == null || endLine == null || startLine === endLine) {
+      hideHint()
+      return
+    }
+    hint.textContent = `Copy context ${formatShortcutLabel(
+      'editor.copyContext',
+      useAppStore.getState().keybindings
+    )}`
+    const rect = selection?.getRangeAt(0).getBoundingClientRect()
+    if (!rect || rect.width === 0) {
+      hideHint()
+      return
+    }
+    hint.style.display = 'block'
+    const above = rect.top > hint.offsetHeight + 12
+    hint.style.left = `${Math.round(rect.left)}px`
+    hint.style.top = `${Math.round(above ? rect.top - hint.offsetHeight - 8 : rect.bottom + 8)}px`
+  }
+
+  // Why: mirrors the editor's selection-clipboard debounce so a drag does not churn X11's buffer.
+  const updatePrimarySelection = (): void => {
+    if (!isPrimarySelectionEnabled()) {
+      return
+    }
+    const text = readSelection()?.toString() ?? ''
+    if (!text || text.length > PRIMARY_SELECTION_MAX_LENGTH) {
+      return
+    }
+    setPrimarySelectionText(text)
+  }
+
+  const handleSelectionChange = (): void => {
+    updateHint()
+    if (primarySelectionTimer !== null) {
+      window.clearTimeout(primarySelectionTimer)
+    }
+    primarySelectionTimer = window.setTimeout(updatePrimarySelection, PRIMARY_SELECTION_DEBOUNCE_MS)
+  }
+
   const handleKeyDown = (event: KeyboardEvent): void => {
     if (!editorShortcutMatches('editor.copyContext', event)) {
       return
@@ -56,8 +128,17 @@ export function installPierreContextualCopy(
     event.preventDefault()
     event.stopPropagation()
     void navigator.clipboard.writeText(formatted)
+    hideHint()
   }
 
   container.addEventListener('keydown', handleKeyDown, true)
-  return () => container.removeEventListener('keydown', handleKeyDown, true)
+  document.addEventListener('selectionchange', handleSelectionChange)
+  return () => {
+    container.removeEventListener('keydown', handleKeyDown, true)
+    document.removeEventListener('selectionchange', handleSelectionChange)
+    if (primarySelectionTimer !== null) {
+      window.clearTimeout(primarySelectionTimer)
+    }
+    hint.remove()
+  }
 }