refactor(changes): dedup the op-preview path and align conventions

- redaction useMemo: one redactText pass per field (the outcome carries
  both text and hit; was a hit-scan plus a mask pass), and the banner
  condition drops the redundant redactionOn check (hit implies on)
- three copy-pasted header toggles -> PaneToggle; the md reading body
  -> MdReadingView (absorbing the mermaid branch + codeLabels)
- redaction preference persisted under the repo's dsh-sidebar:v1: prefix
- render toggle limited to .html/.htm (the editor html viewer's ext set;
  .xhtml went beyond any claim)
- tsx/jsx share TS_WORDS/JS_WORDS; CSS word list loses its duplicate
  entries and its nonexistent true/false constants
- parseReadContent/parseReadLines share the contentLines() envelope core
  (content extraction + Showing-lines filter)

src/client/changes/ops.ts

@@ -201,6 +201,18 @@ export function knownContentBefore(ops: readonly FileOp[], path: string, before:
   return undefined
 }
 
+/**
+ * The read-tool response's `<content>` body split into lines with the
+ * "(Showing lines …)" note dropped — the shared envelope core both read
+ * parsers consume: {@link parseReadLines} recovers line numbers from the
+ * "<n>: " prefixes, {@link parseReadContent} keeps the raw text.
+ */
+function contentLines(raw: string): string[] {
+  const contentMatch = raw.match(/<content>([\s\S]*?)<\/content>/)
+  const body = contentMatch ? contentMatch[1]! : raw
+  return body.split('\n').filter((line) => !/^\s*\(Showing lines .*\)\s*$/.test(line))
+}
+
 /**
  * Strip the DSH read-tool response envelope down to the file's raw content,
  * PRESERVING blank lines (unlike {@link parseReadLines}, which drops them):
@@ -209,11 +221,7 @@ export function knownContentBefore(ops: readonly FileOp[], path: string, before:
  * because markdown block structure depends on them.
  */
 export function parseReadContent(raw: string): string {
-  const contentMatch = raw.match(/<content>([\s\S]*?)<\/content>/)
-  const body = contentMatch ? contentMatch[1]! : raw
-  return body
-    .split('\n')
-    .filter((line) => !/^\s*\(Showing lines .*\)\s*$/.test(line))
+  return contentLines(raw)
     .map((line) => line.replace(/^\s*\d+:\s/, ''))
     .join('\n')
     .replace(/^[\n]+/, '')
@@ -229,12 +237,9 @@ export interface ReadLine { readonly line: number; readonly text: string }
  * "<n>: " prefix as the line number, falling back to sequential counting.
  */
 export function parseReadLines(raw: string): ReadLine[] {
-  const contentMatch = raw.match(/<content>([\s\S]*?)<\/content>/)
-  const body = contentMatch ? contentMatch[1]! : raw
   const result: ReadLine[] = []
   let fallback = 1
-  for (const line of body.split('\n')) {
-    if (/^\s*\(Showing lines .*\)\s*$/.test(line)) continue
+  for (const line of contentLines(raw)) {
     if (line.length === 0) continue
     const match = line.match(/^\s*(\d+):\s?(.*)$/)
     if (match !== null) {