Add custom smart feeds with a rule-based criteria builder

[davidfwatson]

What

Adds the ability to create your own smart feeds, instead of only the three built-ins (Today, All Unread, Starred). A custom smart feed is defined by a set of conditions combined with Match All / Match Any.

v1 conditions: read status (unread/read), starred status, and feed is / feed is not.

The motivating example — "all unread except certain feeds" — is just Match All of [unread, feed is not X, feed is not Y].

How it works

• SmartFeedCriteria (Codable/Sendable/Hashable) lives in the ArticlesDatabase module so both FetchType (Account) and the SQL builder can see it. It builds a bounded WHERE clause. Feed conditions are account-scoped: a feedID from another account reduces to a constant (0/1), so a cross-account smart feed stays correct as each account's database is queried in turn.
• New FetchType.smartFeedCriteria(_:) reuses AccountManager's cross-account fetch aggregation and SmartFeed's per-account unread counting — no new fan-out plumbing.
• UserSmartFeed + CustomSmartFeedDelegate, persisted as JSON in AppConfig.dataFolder. SmartFeedsController now loads/adds/updates/removes them and posts .ChildrenDidChange so both sidebars rebuild; find(by:) resolves custom UUIDs for state restoration.
• A shared SwiftUI editor (SmartFeedEditorView) hosted in a UIHostingController (iOS) and NSHostingController (Mac).
• iOS: Add Smart Feed in the + menu / action sheet; swipe + context-menu edit/delete for custom feeds only (built-ins stay non-deletable); custom removal path since DeleteCommand rejects pseudo-feeds.
• Mac: New Smart Feed… in the File menu and the no-selection sidebar menu; Edit/Delete in the smart-feed context menu, gated to custom feeds.

Testing

• Unit tests (SmartFeedCriteriaTests, 14 cases) cover WHERE-clause building, cross-account scoping, match all/any, negation, the "unread excluding feeds" example, and Codable round-tripping — all passing via swift test.
• Builds cleanly for both the macOS and iOS schemes.
• ⚠️ Still needs in-app manual verification (couldn't be exercised headlessly): create a smart feed, confirm it appears on both platforms with correct contents + unread badge, updates live, persists/restores across relaunch, and edit/delete behave.

Out of scope (possible follow-ups)

Text/body-contains (the FTS index is available), date windows, folder conditions, author/URL matching, reordering custom feeds, and syncing definitions across devices.

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