feat: configurable nudge lock duration per session

[princechrix]

Summary

• Separates session interval from nudge lock duration as two distinct fields
• Session interval (minutes) — how often the nudge fires
• Nudge lock duration (seconds) — how long the overlay stays locked before dismiss is available

Changes

• src/main/sessionManager.js — stores nudgeDuration field in session data
• src/renderer/index.html — new form field with hint text
• src/renderer/styles.css — .field__hint style
• src/renderer/app.js — wires new field for create/edit/display
• src/main/windowManager.js — passes nudgeDuration to overlay
• src/renderer/overlayApp.js — uses dynamic duration instead of hardcoded 5s

Testing

• New field visible in create/edit form
• Duration saved and loaded correctly
• Overlay countdown uses per-session duration
• Backwards compatible with sessions that lack the field (defaults to 5s)
• ESLint passes