feat: scope notifications to deadcat-authored content + topshell polish

[dmnyc]

Summary

The notifications bell was filling up with the user's entire Nostr notifications stream — every reaction, zap, and reply across the network, most of it about non-Deadcat content the user posted from other clients. This PR scopes the bell to deadcat-authored events only via universal NIP-89 client tagging, plus a few topshell polish fixes that surfaced along the way.

Notifications scoping (the headline change)

• Universal NIP-89 client tag on every event the app publishes. Previously only kind:1111 comments carried the tag; now reactions, deletions, follow lists, mute lists, profile updates, wallet backups, market announcements, attestations, pool announcements, limit orders, and order deletions all do too. Centralized via a shared client_tag() helper in the SDK so future builders can't forget it.
• Notification subscription filters target events. The subscription bundles a second filter ({authors: [me]}) alongside the original notifications filter, post-filters incoming events for the deadcat client tag, and tracks the resulting event IDs in a persisted OwnEventsStore (sibling of notifications.json under <app_data_dir>/<network>/). Inbound notifications (kind:7 / 9735 / 1111) are dropped unless their target e tag is in that set.
• Strict filter, no empty-set bypass. Brand-new users with no Deadcat-authored content see an empty bell — correct behaviour; the bell only ever shows reactions/zaps/replies on things they actually posted from this app.
• Profile-level zaps drop too — pubkey-scoped zap receipts can come from any client, so they're indistinguishable from non-Deadcat zaps and stay out of the bell.
• Legacy prune. A scheduled task (10s after subscription start) sweeps the persisted notifications file against the fully replayed own-events set, dropping anything that doesn't target a Deadcat-authored event. Cleans up records that leaked through during the bypass-era without requiring a manual "Mark all read."

Topshell polish

• Two-edge category-bar fade — left + right gradients with hidden scrollbar, conditionally rendered via data-overflow-* attributes computed from scrollLeft / clientWidth / scrollWidth. ResizeObserver + scroll listener keep them in sync. Width 24px, fully opaque at edge — narrow enough to not bleed onto pills, dark enough to read as "more content this way."
• Active pill auto-scrolls into view when activeCategory changes, so a sign-in (which adds "My Markets") or a deep link to a clipped category doesn't leave the new active state hidden.
• Mutually exclusive popovers — opening the notifications bell now closes the user menu and vice versa. No more cross-popover overlap.
• Notifications empty state — capped width + extra padding so the line wraps cleanly inside the popover instead of jamming the edges.

Commits (oldest → newest)

• 4bf8da8 chore(ui): add right-edge fade + hidden scrollbar to category row
• df73773 feat(notifications): scope to deadcat-authored content via NIP-89 client tag
• b472301 fix(notifications): strict deadcat-only filter + persist + prune legacy
• 0fa8567 fix(ui): two-edge category bar fade + tighter notifications empty state
• 0e1975c tune(ui): soften category bar fades so active pills stay readable
• 9e48ac7 tune(ui): narrow category bar fades to 24px so they don't bleed onto pills
• b9350b2 fix(ui): close user menu when bell opens (and vice versa)

Out of scope

• "Show all Nostr notifications" toggle for power users who want one inbox — easy follow-up if requested.
• Mobile responsive overhaul (hamburger / bottom-nav). Tracked in memory; the desktop-narrow scroll fade is the interim affordance.

Test plan

• Bring an existing Nostr identity into the app — bell stays empty until a comment is posted from Deadcat. Existing notifications get pruned within ~10s of session start.
• Post a comment from Deadcat → reaction / reply / zap on it shows in the bell. Reactions on non-Deadcat content do not.
• Profile-level zaps from outside Deadcat don't appear in the bell.
• Category bar shows fades only when there's clipped content past the edge; no fades at full width.
• Clicking a category that's currently clipped (e.g. on a narrow window) auto-scrolls it into view.
• Opening the bell with the user menu open closes the user menu, and vice versa.
• biome check, tsc --noEmit, cargo fmt, cargo clippy --all-targets -- -D warnings all pass.