Contribute rewrite Phase 5: inline word-diff in manual pending card

The §8.11 manual-edit card was a stub — "{N} paragraphs edited
directly" plus a countdown and a Save-now button. The spec says the
card should grow one inline word-diff per touched paragraph as the
contributor types, in the same green/red register the Phase 3 accepted
overlay uses. Phase 5 lands that.

In RFCView.jsx, the 800ms-debounced handleEditorUpdate now computes a
per-paragraph token diff alongside the existing paragraph count and
stores both on manualPending. Baseline is the same Phase 4 gutter
source — originalSourceLinesRef.current, the last server-confirmed
body split on blank lines — so the card resets to empty the same
moment the gutter clears (accept/decline/manualFlush/branch-switch).

In ChangePanel.jsx, diffWords is exported (it was already the AI
card's inline-diff engine — token-level LCS over a whitespace-
preserving /(\\s+)/ split, ~30 lines, no runtime dep). The manual
card consumes the same tokens. Wholly-inserted paragraphs render as
all-insert blocks; wholly-deleted paragraphs as all-delete blocks.

Visual register is intentionally shared with Phase 3's preview
overlay: same #dcfce7/#166534 inserts, same #fee2e2/#991b1b
strike-through deletes. Selectors are scoped under .change-manual-diff
rather than reusing .markdown-preview .tracked-* since the card lives
outside the preview surface.

Pre-fancy stance, matching Phase 4's gutter: the diff is index-aligned
against the baseline, so adding a paragraph in the middle lights up
the rest of the doc. Tolerated as the "you've touched stuff below this
point" cue. An LCS-anchored future pass can fix it.

Verification gap matching Phases 2/3/4: backend was not running this
session, so the live RFCView → real-branch flow wasn't exercised.
Drove the Vite preview sandbox instead — mounted ChangePanel with a
hand-built diffs payload, confirmed three blocks render (mixed edit,
wholly-inserted, wholly-deleted), inserts/deletes carry the expected
computed colors, no console errors. Backend integration suite still
green (125 passed).

frontend/src/components/ChangePanel.jsx

@@ -11,7 +11,10 @@ import { useState, useEffect, useRef } from 'react'
 
 const PREVIEW_LENGTH = 220
 
-function diffWords(original, proposed) {
+// Word-level LCS over whitespace-preserving tokens. Used by both the
+// AI change-card InlineDiff (original→proposed) and Phase 5's
+// manual-pending-card per-paragraph diff (baseline→current).
+export function diffWords(original, proposed) {
   const a = (original || '').split(/(\s+)/)
   const b = (proposed || '').split(/(\s+)/)
   const m = a.length, n = b.length
@@ -85,6 +88,7 @@ export default function ChangePanel({
         {manualPendingStatus && (
           <ManualPendingCard
             paragraphCount={manualPendingStatus.paragraphCount}
+            diffs={manualPendingStatus.diffs}
             savingIn={manualPendingStatus.savingIn}
             onSaveNow={manualPendingStatus.onSaveNow}
           />
@@ -114,7 +118,7 @@ export default function ChangePanel({
   )
 }
 
-function ManualPendingCard({ paragraphCount, savingIn, onSaveNow }) {
+function ManualPendingCard({ paragraphCount, diffs, savingIn, onSaveNow }) {
   return (
     <div className="change-item type-manual state-pending">
       <div className="change-meta">
@@ -124,6 +128,19 @@ function ManualPendingCard({ paragraphCount, savingIn, onSaveNow }) {
       <div className="change-label">
         {paragraphCount} paragraph{paragraphCount === 1 ? '' : 's'} edited directly
       </div>
+      {diffs && diffs.length > 0 && (
+        <div className="change-manual-diff">
+          {diffs.map(d => (
+            <div key={d.baselineIndex} className="change-manual-diff-block">
+              {d.tokens.map((t, idx) =>
+                t.type === 'same'   ? <span key={idx}>{t.text}</span> :
+                t.type === 'add'    ? <span key={idx} className="manual-diff-insert">{t.text}</span> :
+                                      <span key={idx} className="manual-diff-delete">{t.text}</span>
+              )}
+            </div>
+          ))}
+        </div>
+      )}
       <div className="change-manual-status">
         unsaved · auto-save in {savingIn}
         <button type="button" className="btn-save-now" onClick={onSaveNow}>Save now</button>