feat(desktop): per-event notification sounds and alert controls

[tellaho]

Category: new-feature
User Impact: Users can now choose a notification sound — or a different one per event type — preview each sound from a waveform picker, and toggle alerts on or off per event, including new notifications for thread replies and for DMs in the conversation they have open.

Problem: Notifications were one-size-fits-all: a single hardwired sound, no control over which event classes alert, no notifications at all for replies in your threads, and DMs always suppressed for the conversation being viewed.

Solution: The Notifications settings are rebuilt around per-event rows — each event class gets its own alert toggle and sound picker that inherits a top-level default until overridden, with waveform thumbnails generated from the actual audio so sounds are recognizable at a glance. Two new notification classes ship with it: thread replies (threads you authored, participated in, or follow) and an opt-in "Notify while viewing" for open DMs. Job-lifecycle rows are wired end-to-end but render as "coming soon" behind a View-all toggle, since nothing emits job events (43001–43006) yet.

File changes

*desktop/public/sounds/*.mp3, .svg
Eleven new notification sounds plus the renamed original (flutter). Each ships with a waveform SVG generated from its PCM data, used as the picker thumbnail (fill="currentColor" so they tint with the theme).

desktop/scripts/generate-sound-waveforms.mjs
Generator for the waveform SVGs: ffmpeg decode, energy-based tail trim, narrow-window sampling, and an unsharp-mask pass so each sound's silhouette reads distinctly. Run with node scripts/generate-sound-waveforms.mjs public/sounds.

desktop/src/features/notifications/lib/sound.ts
The sound model: sound names, event slots (SoundSlot), per-slot labels/descriptions/recommendations, per-slot overrides with resolveSlotSound (override ?? default), per-slot alert enablement defaults, and the coming-soon slot set. Audio playback moves to a per-sound element cache.

desktop/src/features/notifications/hooks.ts
NotificationSettings gains singleSound, per-slot sounds overrides, slotAlertsEnabled, and notifyWhileViewing; the old top-level mentions/needsAction booleans migrate into their slots via the sanitizer, so stored settings survive without a version bump.

desktop/src/features/notifications/use-feed-desktop-notifications.ts
Feed-driven notifications (mentions, needs-action) respect the per-slot alert toggles and play the per-slot resolved sound.

desktop/src/features/channels/useLiveChannelUpdates.ts
Two new dispatch options: notifyForActiveChannel (DM gate bypass for the open conversation) and onThreadReplyDesktopNotification, fired for replies in authored/participated/followed threads (non-DM channels; same active-channel rule as DMs).

desktop/src/app/AppShell.tsx
Handlers for the two new notification classes — thread replies (skipping @-mention replies, which the mention path owns) and DM-while-viewing — plus per-slot sound resolution at both play sites and settings prop wiring.

desktop/src/features/settings/ui/NotificationSettingsCard.tsx
The rebuilt panel: Desktop alerts + Notify while viewing; a Sound row (default picker + master toggle) with per-event rows as flat siblings below a divider; per-row picker + alert toggle; parent toggles cascade by fading rows in place; coming-soon rows behind a centered View all / Show less button.

desktop/src/features/settings/ui/SoundPicker.tsx
New dropdown picker: waveform thumbnails per sound, recommended marker, a "Default · {sound}" inherit option, and a play/pause preview button.

desktop/src/features/settings/ui/SettingsOptionGroup.tsx
Groups restyled to the member-profile look — muted fill, rounded corners, no borders — shared across all settings panels.

desktop/src/features/settings/ui/SettingsPanels.tsx, SettingsScreen.tsx, SettingsView.tsx
Prop threading for the new setters.

Reproduction Steps

1. Open Settings → Notifications. The sound section shows a Sound row with a waveform picker and master toggle, and one row per event type below a divider.
2. Click any ▶ to preview a sound (icon swaps to pause while playing). Open a picker — each option shows its waveform, with the recommended sound marked rec. and a "Default · {sound}" inherit option on event rows.
3. Change the top-level Sound — every row still on Default follows it.
4. Toggle an event row off — its picker fades and that event stops alerting. Toggle the Sound master off — all rows fade and alerts stop; flipping it back restores each row's stored state.
5. Click "View all" below the container to reveal the four disabled coming-soon job rows.
6. Thread replies: from a second account, reply in a thread you started while you view a different channel — a "Reply in {channel}" notification fires with your default sound.
7. Notify while viewing: enable the toggle, then receive a DM in the conversation you have open — it now notifies (off restores the old suppression).

Screenshots/Demos

(to be added before marking ready for review)