Cross-window sync, global search, and a Flatpak theme fix
- Lists and tasks created or edited in one window now sync automatically to every other open window
- Add a global search box, always visible in the header, that searches every list's tasks at once and groups results by list
- Search field placeholder is now context-aware ("Search <list>") instead of the generic "Search tasks"
- Search bar moves below the title instead of overlapping it when the window is condensed
- "Hide completed" is now a single toggle in the View menu instead of a per-list checkbox
- Add Ctrl+F (toggle search) and Ctrl+H (toggle hide-completed) shortcuts
- Fixed Ctrl+I: "Set icon" and "About" silently collided on the same shortcut, so "Set icon" never fired; rebound to Ctrl+Shift+I
- Delete button in the task edit pane is text-only, no redundant icon
- Notes editor border and background now match the rest of the app's text fields
- Toasts now close when dismissed instead of lingering
- Fixed a long-standing bug where the Flatpak build never picked up system dark/light theme changes; switches are now detected instantly, matching native builds
