docs: document widget refresh budget constraints

[rudeus112266]

Summary

Adds docs/widget-refresh-budget.md documenting the OS-level refresh budget constraints WidgetKit (iOS) and Glance (Android) impose on widgets, independent of what the app requests.

Context

TTLWidget.swift's computeNextUpdateInterval already scales the requested refresh interval by vault urgency (15 min normal → 2 min critical), but that behavior wasn't documented against the actual platform ceiling. This doc gives whoever picks up #198/#244/#248 the real-world numbers up front:

• WidgetKit's observed daily reload budget and how it's silently coalesced when exceeded
• How TTLWidget's urgency scaling spends that budget on the windows that matter rather than assuming unlimited refreshes
• Glance's ~30-minute periodic-update floor for future Android widget work

Testing

Documentation-only change; no code paths affected. Verified references against the current TTLWidget.swift implementation.

Closes #303

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