feat: Full StatusNotifierItem (SNI) tray implementation

[miramir]

What

• Full SNI properties: Category, Status, Id/ItemId, Title, AttentionIcon*, OverlayIcon*
• Item status handling: Passive items are now hidden (per spec); NeedsAttention items display attention/overlay icons
• Context menu, secondary activate, scroll: wired D-Bus methods + ItemsRemoved signal handling for dbusmenu
• Late-start self-healing: discovery poll reduced from 30s → 3s; watcher re-reads RegisteredItems on each tick to catch clients that registered before the watcher existed
• Better icon selection: best_icon_pixmap() picks the largest pixmap; current_icon_from_proxy() falls back through pixmap → icon name → XDG theme
• Tests: unit tests for ItemStatus, best_icon_pixmap, pixmap_to_icon, split_service_name, menu helpers (is_separator, is_visible, renderable_children), is_blocklisted, XDG icon normalization
• Docs: website tray page updated with SNI protocol details; D-Bus interface reference lists com.canonical.dbusmenu
• .gitignore: added .vscode/

Breaking change

Passive SNI items are now hidden from the tray (previously shown alongside Active). Matches the SNI spec