Add configurable periodic sync & discovery; improve settings UX; sync only watched games

[danielphingston]

Motivation

• Give users control over background behavior by making periodic R2 sync and game discovery independently configurable and user-friendly.
• Avoid surprising UI/OS behavior by preventing the Windows folder picker from briefly showing a terminal window.
• Improve discoverability and clarity of settings so schedules and device controls are easier to understand and change.

Description

• Introduced an Automation section in config with PeriodicSyncEnabled, SyncIntervalMinutes, PeriodicDiscoveryEnabled, and DiscoveryIntervalMinutes, plus DefaultAutomation() and validation in internal/config/config.go and migration for older configs.
• Added ConfigureAutomation to the settings manager (internal/settings/manager.go) and a new HTTP endpoint PUT /api/settings/automation with server handling in internal/api/server.go and tests in internal/api/server_test.go.
• Added UI for automation on the Settings page (internal/api/web/app.js, internal/api/web/styles.css) including toggles, interval inputs, explanatory copy, and an advanced disclosure for launcher paths; settings apply live without restart.
• Periodic sync now runs from the coordinator loop (internal/app/coordinator.go) on a one-minute ticker, using configured intervals; discovery runs independently on its own configured interval.
• Periodic sync is limited to watched games only: new DB query PendingSnapshotsForWatchedGames and helpers in internal/database/store.go, and coordinator now syncs watched pending snapshots instead of all pending snapshots for periodic runs while leaving manual SyncPending unchanged.
• Prevented the Windows folder picker from flashing a PowerShell terminal by launching it with hidden-window flags (internal/platform/folder_windows.go).
• Added unit tests and integration-style checks for new behavior and persistence (internal/config, internal/settings, internal/database, internal/api tests) and updated README to document the new options.

Testing

• Ran go test ./... and go test -race ./... and all package tests passed (including new/updated tests in internal/config, internal/settings, internal/database, internal/api).
• Ran make check (format/lint/tests) and it completed successfully after adjustments to interfaces and constructor wiring.
• Verified Windows folder picker behavior via GOOS=windows GOARCH=amd64 go test ./internal/platform ./cmd/saveknot (tests passed in this environment).
• Exercised the Settings UI with agent-browser to open the Settings page, interact with the automation form, save values, and capture a screenshot; the UI flow completed successfully.
• Note: an attempt to call a local make_pr helper to open a remote PR failed because make_pr is not available in this environment and no remote is configured. No remote Git/GitHub operations were performed.

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Codex Task