feat(notifications): add overlay notifications with position config (#110)

docs/plans/2026-06-09-macos-notification-hover-design.md

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+<!-- read_when: changing overlay notification hover, macOS mouse passthrough, or notification actions -->
+
+# macOS Notification Hover Stability Design
+
+Status: approved
+Date: 2026-06-09
+
+## Problem
+
+On macOS, hovering a character dictionary build notification can make the card flicker and slide as
+if it is hiding, then snap back. The likely trigger is the notification stack changing the overlay
+window's mouse-passthrough state for a progress card that has no user action.
+
+## Chosen Approach
+
+Keep non-action overlay notifications visually stable and click-through on hover. Only notifications
+with explicit actions should request interactive overlay input. The notification history panel keeps
+its existing interactive behavior.
+
+This avoids a macOS mouseenter/mouseleave passthrough loop for passive progress cards while
+preserving clickable notification actions.
+
+## Checks
+
+- Add a renderer regression test for passive notification hover.
+- Keep action-bearing notification cards interactive.
+- Run the targeted overlay notification and mouse-ignore tests.