Consolidate on-device error logger for debug builds

[tabmail-kmyi]

Right now, it's set to multiple log files, but they should really be a single file. One for main app, and one for NSE.

[tabmail-kmyi]

Deferred by owner decision (2026-08-26). PR #86 stays open; nothing is waiting on it.

Reasoning: the change is diagnostics-only — no user data, no behavioural contract — so it is not
part of the current correctness work package. It is also the largest non-functional diff in the
open set (+3725/−571 across 20 files), and it introduces a startup migration that runs on every
existing install, so it deserves its own review pass rather than riding along with unrelated work.

Picking it up later costs nothing: the branch is pushed and the tests are written.