feat: add mute-when-focused toggle (+ make-dev watcher fix)

[StuBehan]

Summary

• Surfaces the existing frontmost-window suppression as a configurable toggle (`STACKNUDGE_MUTE_WHEN_FOCUSED`, default `true` to preserve current behaviour). When off, banners + voice fire regardless of which window is focused.
• Wires the toggle through `notify.sh`, the panel's Settings page (new row in the Toggles section), and the menu-bar dropdown.
• Fixes `make dev`'s watcher: `notify.sh` and `phrases/` were in `WATCH_DIRS` but the find filter was scoped to `.swift` / `Info.plist`, so shell-script edits never triggered a reload. Adds `.sh` to the filter and drops the `notifier/` dir (its sources were removed in the single-binary refactor — only `notifier/Icon.icns` is still used at build time).

Test plan

• `make build` — single-binary app compiles clean (arm64)
• `shellcheck --severity=warning notify.sh` passes
• `bash -n notify.sh` passes
• Manually toggle `Mute when focused` from the panel UI and confirm banner/voice fire when off, suppress when on
• Manually toggle from the menu-bar dropdown and confirm parity with panel UI
• Edit `notify.sh` while `make dev` is running, confirm reload triggers