feat(app): add multi-provider visual distinction and connected accounts UI

[mschilling]

Description

With Azure DevOps now supported alongside GitHub, the frontend needs UI enhancements to visually distinguish between providers and manage connected accounts.

Current State

The frontend is already provider-agnostic — no hardcoded GitHub assumptions exist. Activities, leaderboards, and profiles all work with any provider. However, there's no way for users to see which provider an activity came from or manage their connected accounts.

Requested Enhancements

1. Provider Badge on Activity Cards (Medium Priority)

• Add a small provider indicator (GitHub icon, Azure DevOps icon) on activity cards in the Activity Wall
• Could be a subtle icon next to the repository name or timestamp
• File: apps/frontend/app/src/app/components/activity-item.component.ts
• The provider field is already available in the activity data

2. Connected Accounts Section on Profile (Medium Priority)

• Show which providers the user has linked (GitHub, Azure DevOps, Strava, etc.)
• Display provider-specific info (username, avatar)
• File: New component apps/frontend/app/src/app/pages/profile/components/connected-accounts.component.ts

3. Provider Filter on Activity Feed (Low Priority)

• Add filter chips (All / GitHub / Azure DevOps) to the Activity Wall
• Let users view activity from a specific provider
• File: apps/frontend/app/src/app/pages/activity-wall/activity-wall.component.ts

4. Provider Filter on Leaderboard (Low Priority)

• Option to show rankings by individual provider vs. aggregate
• File: apps/frontend/app/src/app/pages/hq/components/leaderboard-modal.component.ts

Notes

• All activity data already includes the provider field from backend
• Provider types are defined in libs/types/src/lib/core/providers.ts
• No backend changes needed — this is purely frontend work