fix(sync): propagate settings decode errors instead of silently skipping

[datlechin]

Summary

• SyncCoordinator.applySettingsData(_:for:) is now throws and lets decode failures propagate as SyncDecodeError.decodeFailure(field:underlying:). The eight category branches (general, appearance, editor, data grid, history, tabs, keyboard, AI) drop the per-branch if let settings = try? decoder.decode(...) { ... } pattern in favor of a single try inside a shared do/catch.
• applyRemoteSettings(_:) wraps the call in do/catch and logs the failing record name + category + error, then skips the record.

Removes 32 lines of if let try? boilerplate while making the failure path observable.

Why this matters

This is bug B3 from the full-app audit. Same anti-pattern as #1141 (B2), one layer up. A settings record written by a newer schema version would deserialize to nil, the if body would be skipped, and the user would think their settings synced when they hadn't: no log, no error, no UI signal. After this PR the failure is logged with the failing category name, and the record stays unapplied so a future build can pick it up.

Test plan

• Manual: write a CloudKit settings record with a malformed settingsJson blob for one of the eight categories, run a pull, confirm the log line Skipping remote settings ... (general): Sync record decode failed for general: ... and that local settings are unchanged.
• Manual: confirm a normal pull with valid settings still applies them and advances state.
• swiftlint --strict clean.

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