refactor(desktop): consolidate notification helpers and add channel names to toasts

[wpfleger96]

What

Extracts three shared helpers into a new notificationFormat.ts module and wires all desktop notification paths through them, fixing missing channel names in thread reply and reminder toasts.

New file: desktop/src/features/notifications/lib/notificationFormat.ts

• resolveNotificationChannelLabel(channelId, channels) — returns "#channelName" or null (null-safe: returns null when the channels query hasn't resolved yet, so the toast still fires with just the prefix)
• truncateNotificationBody(content, fallback) — consistent 140-char truncation with ... suffix
• formatNotificationTitle({ prefix, channelLabel }) — produces "prefix in #channel" or just "prefix"

Why

Thread reply toasts fell back to "Reply in Thread" when the channels array was empty at WebSocket event time (race condition: the channels query hadn't resolved yet). Reminder toasts had no channel context at all. Each of the four toast paths had its own inline body truncation logic with slightly different behavior.

Changes per path

Path	Before	After
Feed (mentions/needs_action)	Own notificationTitle() + inline truncation	Delegates to formatNotificationTitle() + truncateNotificationBody(); behavior unchanged
Thread replies	Inline channels.find() → "Reply in Thread" on miss	channels.find() → "Reply in #channelName" or "Reply" on miss
DMs	Inline truncation	Uses truncateNotificationBody(); title unchanged (participants are the title)
Reminders	Hardcoded "Reminder due"	"Reminder due in #channelName" for single-reminder toasts with a target channel

Notes

• The race condition fallback is intentional: returning null from resolveNotificationChannelLabel when the channel isn't in the list means the toast fires with just the prefix rather than being blocked. This is better UX than a silent miss.
• Multiple due reminders omit channel context (they may span different channels); the body count "N reminders are due" is the signal.
• useChannelsQuery() in AppShell.tsx is moved 13 lines earlier so channels is available before useReminderNotifications is called. Hook call order is preserved.