feat(changes): secret redaction layer for op previews

The changes-tab op previews replay read/write/edit payloads into the DOM
(diff rows, read rows, markdown reading mode, error text) — when the model
reads a credentials file, that plaintext must not land on screen or in
inspectable DOM. New display-only redaction layer, on by default with a
one-click pane toggle (persisted in localStorage):

- path layer: credential-shaped filenames (.env / *secret* / *credential* /
  *token* / *api-key* / private keys...) mask whole files line-wise
- content layer: ordinary files mask only secret-shaped spans — secret
  field assignments keep the field name (api_key: [REDACTED]), plus bare
  token prefixes (sk- / AKIA / ghp_ / xox / ya29.), Bearer headers, and
  PEM private-key block headers
- redacted payloads flow through the same viewOp shape into diff rows,
  read rows, markdown reading mode, and error text

Known edge (documented in README): an unquoted mermaid node label
containing a redacted secret breaks the diagram and falls back to source;
workaround is quoting the label. Session data is untouched — this is
display-only and not a boundary against same-context malicious plugins.

Locales: zh/en dictionaries + all 19 shipped third-language overrides
(key-set-equal gate green). 9 new unit tests; typecheck clean; targeted
suites 40/40 green. The fs-operations failure is pre-existing on the base
branch (environment-dependent, verified via stash) and unrelated.

src/client/changes/changes.module.css

@@ -342,6 +342,7 @@
 
 /* ── Shared preview pane ───────────────────────────────────────────────── */
 
+.redactBanner { flex: none; font: var(--dsw-font-xxxs-strong-11); padding: 2px 8px; border-radius: 7px; color: var(--dsw-alias-state-warning-primary, #b8860b); background: var(--dsw-alias-interactive-bg-hover); white-space: nowrap; }
 .mdToggle { flex: none; border: 1px solid var(--dsw-alias-border-l1); border-radius: 7px; background: transparent; color: var(--dsw-alias-label-secondary); font: var(--dsw-font-xxxs-strong-11); padding: 2px 8px; cursor: pointer; }
 .mdToggle:hover { background: var(--dsw-alias-interactive-bg-hover); color: var(--dsw-alias-label-primary); }
 .mdToggle[data-on='true'] { border-color: var(--dsw-alias-state-business-primary); color: var(--dsw-alias-state-business-primary); }