Add manual feed reordering, synced via iCloud (#760)

[dcaspi]

I've been an avid NetNewsWire user for several years and love just about
everything about it. The one feature I've kept wishing for is the ability
to order feeds, so the ones I care about stand out from the noise — so I
decided to help. I've tried to stick to the idioms already in the codebase,
keep the change minimal while still working well, and add tests.

Resolves #760.

What

Feeds in the sidebar can now be reordered by dragging them within their
container (an account's top level, or a folder) — on both Mac and iOS. You
can already drag feeds today; previously the drop just snapped back. The
order is per-feed and syncs across devices on an iCloud account.

How

• Model — a per-feed sortIndex: Int (default 0), stored in a new
  sortIndex column on FeedSettingsDatabase (with an idempotent
  ALTER TABLE migration for existing databases), exposed via
  FeedSettings.sortIndex / Feed.sortIndex — the same pattern as
  editedName, externalID, etc.
• Sidebar order — a new Node comparator orders feeds by
  (sortIndex, name); folders still come last, alphabetically.
  SidebarTreeControllerDelegate switches to it — one change, both
  platforms. Because every feed defaults to sortIndex 0, the name
  tie-break reproduces today's alphabetical order exactly: no migration
  step, and no behavior change for anyone who never drags a feed.
• Reorder action — Account.reorderFeeds(_:in:) assigns 0..<n,
  posts ChildrenDidChange, and calls a new AccountDelegate.reorderFeeds
  (a default no-op; only CloudKitAccountDelegate overrides it — the
  sortIndex setter has already persisted locally for every account type).
• iCloud sync — a new sortIndex field on the AccountWebFeed CloudKit
  record. CloudKitAccountZone.saveSortIndexes(for:) writes it with a
  changed-keys merge (other fields untouched, same as renameFeed);
  CloudKitAccountZoneDelegate reads it back on incoming records.
• Drag and drop — extended the existing Mac (SidebarOutlineDataSource)
  and iOS (MainFeedCollectionViewController+Drop) handlers: a feed dropped
  at a new position within its own container now calls reorderFeeds.
  Shared helpers: Array<Feed>.reordered(moving:before:) and
  Container.topLevelFeedsInDisplayOrder().

Scope

v1 reorders feeds — at the top level and inside folders. Folders themselves
still sort alphabetically at the end. Possible follow-ups: reordering
folders (needs a Folder sortIndex + an AccountContainer CloudKit field);
placing newly-added feeds at the bottom of a manually-ordered container
(today they default to 0, so they land near the top); a "Reset to
alphabetical order" command.

Tests

New XCTest coverage in AccountTests: the sortIndex column + migration
from a pre-sortIndex schema, the Feed/FeedSettings proxy + change
notification, Account.reorderFeeds, and Array<Feed>.reordered. All 105
AccountTests pass; both app targets build. Manually verified drag-reorder
(top level and in folders, both platforms) and iCloud sync between devices.

Note on the CloudKit schema

The new sortIndex field on AccountWebFeed auto-creates in the CloudKit
development environment on first write; it'll need a schema deploy to
production before a release. Adding a field is backward compatible — older
clients just ignore it.

🤖 Generated with Claude Code