Load local analytics only when needed

[thrr87]

What

• Keep the default Usage remaining view on account data only.
• Load local Codex records only when a local graph, Facts, or Insights needs them.
• Save new local facts and release the in-memory cache when the user leaves those views.
• Keep automatic refreshes from restarting a local import.
• Reduce the Usage remaining chart height from 300 to 240 points.

Why

A real 400 MB Codex history can produce more than 400,000 local facts. The
default view did not need them, but opening the app still loaded them into
memory.

This is the first part of #36. It removes that work from the common path.
The next stacked change will replace the remaining full in-memory local
history with bounded SQLite queries.

Measured on the real profile

• Clean QA launch on Usage remaining: about 102–103 MB RSS.
• Manual account refresh on that view: about 103 MB RSS.
• Local Facts still reaches about 160–175 MB and needs the SQLite step.

Checks

• 500 Swift tests pass.
• Added race tests for hiding a local view during a suspended read.
• Verified Graphs, Facts, and Insights with the full-screen QA app.
• The shorter chart keeps all Usage remaining controls and facts visible
  without a scroll.

Stacked on #33.