test: cover ui/chart.rs and app.rs state transitions

[kleyt0n]

Problem

src/ui/chart.rs is 372 lines with 0 tests. src/app.rs is 1138 lines with 6 — the thinnest ratio in the tree, against src/ui/mod.rs at 47/3179 — and it holds the entire state machine: move_bucket, clear_bucket, toggle_focus, cycle_granularity, scroll_pane, select_session_row.

Why it matters

Every keypress goes through app.rs, and #22 will add a scan-swap path that has to preserve all of that state. Testing it is a prerequisite for changing it safely.

What to do

• Chart bucketing and the other bucket at MODEL_SLOTS = 6.
• State transitions in app.rs, including bounds — moving past the last bucket, clearing an already-clear bucket, cycling granularity round the loop.
• Apply the repo's existing arithmetic-test convention: regrouping a chart by day, week or month must not change the grand total (docs/guide/building.md:76-85).

Acceptance

Both files have coverage of their branch logic, and a granularity change is proven total-preserving.