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

docs/plans/2026-06-06-early-managed-overlay-startup-design.md

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+<!-- read_when: changing managed mpv startup, pause-until-ready, or visible overlay boot ordering -->
+
+# Early Managed Overlay Startup Design
+
+Status: approved
+Date: 2026-06-06
+
+## Problem
+
+Managed mpv startup can pause playback immediately, then leave SubMiner's tray and visible overlay
+unavailable until Yomitan/tokenization warmups finish. Startup notifications therefore miss the
+overlay surface and fall back to non-overlay status paths.
+
+## Chosen Approach
+
+For cold `--start --background --managed-playback` launches, handle initial args before waiting for
+the deferred overlay warmup. That lets the tray and visible overlay shell initialize immediately
+while the existing tokenization warmups continue in the background.
+
+The mpv plugin pause gate stays armed. Playback release still waits for SubMiner's autoplay-ready
+signal, which is emitted only after tokenization warmup and visible-overlay readiness. Existing
+second-instance attach behavior remains unchanged: when the launcher finds an already-running
+background app, it sends the same control command to that process and reuses its warmups/tokenizer.
+
+## Checks
+
+- Add a startup ordering regression test for managed background playback.
+- Keep the existing deferred startup ordering for non-managed launches.
+- Run the startup/runtime test slice plus SubMiner verification lane.