feat(settings): change card detail control in CheckpointSettings + webview default (B3a v2-9, epic #1375)

[easonLiangWorldedtech]

What this PR does

Settings/UI tail of the B3a change-card feature (epic Zoo-Code-Org#1375), stacked on #51 (v2-8b):

• CheckpointSettings: new "Change card detail" control (summary / full) bound to local cachedState and saved through the updateSettings payload (changeCardDetail), per the SettingsView cache pattern.
• SettingsView: wiring for the new control; changeCardDetail added to the settings cache.
• ExtensionState: changeCardDetail defaults to "summary" in createInitialExtensionState; mergeExtensionState preserves the stored value when the incoming state omits the key.
• i18n: settings:checkpoints.changeCardDetail.{label,description} in all 18 locales (nested under checkpoints, sibling of perWrite).
• Specs: B3a-parity CheckpointSettings / SettingsView specs + ExtensionStateContext spec (pre-hydration defaults, merge retention with non-default fixtures).

Line count / split rationale

355 changed lines (337+/18-) across 24 files — below the 400 soft design target. The file count is inflated by the 18 locale JSONs (each +4 lines for the two new keys); the actual logic change is the settings control wiring plus the context default/merge handling.

Not included here (later v2 PRs)

• Rollback core + service — v2-10 / v2-11.
• Rollback UI + remaining epic work — v2-12..v2-15.

Local gates

• Vitest (3 touched suites): 57 passed (including 2 new CheckpointSettings assertions pinning the searchable label and the rendered description).
• ESLint --prune-suppressions --max-warnings=0 on the 6 TS/TSX files: clean.
• Prettier: all 24 files content-clean under the repo prettier config (LF-normalized API audit).
• Stryker diff gate: 14 mutants — 14 Killed, 0 Survived, 0 NoCoverage (run against pre-rebase base e6133ebed; diff content unchanged by the rebase onto 49452076d).

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