feat: Redesign Sync Monitor and Enforce Provider Configs

[hendripermana]

Description

This PR implements visual improvements and functional enhancements for the Sync Monitor and Provider configurations.

Key Changes

• Sync Monitor Redesign: Updated the layout and aesthetics to strictly follow the DESIGN.md guidelines.
  • Restructured into a clean section-card pattern.
  • Implemented large pill-rounded geometry (rounded-[24px]) for summary cards.
  • Changed text status indicators into colored pill badges (e.g. badge-positive, badge-negative).
  • Standardized buttons with pill shapes.
  • Removed Tailwind's divide-y to fix a 1px layout jitter bug during hover states, falling back to explicit borders.
• Provider Credentials Enforcement: Implemented logic so that if a provider (Plaid, Lunchflow, SimpleFIN) lacks valid credentials, the user cannot enable the connection. If toggled on, it will automatically toggle off if unconfigured.
• Improved Controller Logic: SyncMonitorsController#show now reliably lists only the latest status for each target instead of a messy raw history, using PostgreSQL's DISTINCT ON.

Validation

• Brakeman shows 0 security vulnerabilities.
• Controller and Model tests pass successfully.