feat(desktop): coalesce burst feed notifications into one toast

[mfethe1]

A burst of feed items produced one OS notification and one sound per item, so ten items meant ten toasts.

This buffers eligible feed items for a bounded 2s window and flushes once:

• 2+ items collapse into a single count-only summary
• a lone item keeps the existing single-item format, so the common case is unchanged

The summary is derived only from counts and the relay's existing needs_action partition — no item content, channel name, or pubkey reaches the toast.

Scope: the feed producer only. The WebSocket producer is deliberately untouched, so an item arriving via WS during an open window can still raise its own toast. That is a known, deliberate limitation of this slice rather than an oversight — keeping it narrow avoids colliding with #5806, which owns hooks.ts and app/useAppShellDesktopNotifications.ts.

3 files, +252/-1. New logic is a pure module (lib/coalesce.ts) with 6 unit tests.

Checks

• pnpm typecheck — clean
• pnpm check:px-text, check:pubkey-truncation — clean
• notifications suite — 79/79
• full desktop suite on this branch's base — green

pnpm check:file-sizes is red on main today for ChannelPane.tsx / ChannelScreen.tsx; this PR touches neither and adds no lines to either.

Not verified (stated explicitly)

• No screenshots. The change affects macOS OS-level notification toasts, which need an interactive launch with notification permission to capture. I could not produce visual evidence, so the rendered toast is unverified by image.
• No fake-timer test for the buffer/flush timer path. The feature has no React-hook test harness (every sibling test is a pure module test), so the timer path is covered by typecheck and by the pure-module tests around it, not by a test that advances time. That is the weakest point of the change and the place most worth reviewer attention.

Overlap

#5806 also touches use-feed-desktop-notifications.ts (open, last updated 2026-08-22). It does not add lib/coalesce.ts. No open PR implements notification coalescing/digest policy — the other notification PRs are delivery-layer (sound, toast delivery, permission, click routing). Happy to rebase if #5806 lands first.