feat: Add the FDv2 connection modes to the client

[beekld]

Summary

Defines the client's FDv2 connection modes and assembles the data sources each one uses.

• Adds a ConnectionMode enum and a client FDv2Config describing each mode's data sources.
• Adds the desktop defaults: three modes, starting in streaming, with automatic mode switching off.
• Adds BuildModeSources, which turns a mode's config into the initializer and synchronizer factories it needs.
• Seeds the polling synchronizer's first-poll time from the cache's per-context freshness, so relaunching does not produce a burst of requests.