Keep the Leveled Reader control block visible while the stats scroll (BL-16585)

[StephenMcConnel]

The Decodable Reader tool keeps its stage nav pinned at the top and its "Book is Decodable" / "Set Up Stages" control block pinned at the bottom, letting only the word lists scroll. The Leveled Reader had no internal scrolling region, so a tall stats panel made the whole tool overflow: the accordion panel scrolled and the "Book is Leveled" toggle and "Set Up Levels" button scrolled out of sight.

This gives the Leveled Reader the same three-band layout:

	pinned top	scrolls	pinned bottom
Decodable	Stage n of m	grapheme + word grids	Generate Report / toggle / Set Up Stages
Leveled	Level n of m	banner, legend, stats, "For this Level", "Keep in mind"	Copy Book Stats / toggle / Set Up Levels

The scrolling region carries a min-height floor (as the Decodable Reader's sections do) rather than min-height: 0. Without a floor it is the only shrinkable item in the column, so in a short panel it collapsed to zero height, leaving the stats both invisible and unscrollable. With the floor, a panel too short even for that overflows and scrolls as a whole — what it did before this change, and what the Decodable Reader does today.

CSS only; no behavior or API changes.

Verified in a running Bloom

Measured over CDP in the Edit tab with a leveled book, at several forced panel heights:

• Normal height: the stats region scrolls internally; nav and control block stay put.
• Squeezed to 220 / 170 / 140 px: the stats region floors instead of collapsing, the panel scrolls as a whole, and every control (including "Set Up Levels") remains reachable — the same numbers the Decodable Reader produces under the identical squeeze.

Ref: https://issues.bloomlibrary.org/youtrack/issue/BL-16585

Devin review

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This change is 

[StephenMcConnel]

[Claude Opus 5, during preflight] Consulted Devin on 2026-08-05 17:34 MDT, up to commit 2b5d35a.

No bugs. One Investigate flag — that the 120px floor on the scrolling region is a magic number — which is now its own thread on the diff, assessed as not an issue and resolved there with the reasoning.

Five informational items, not mirrored as threads. Two of them simply confirmed the layout reasoning (that dropping margin-top: auto is safe now that the scroll region absorbs the slack, and that the nav can't be squeezed). One noted that the whole-panel fallback scrolling was relying on overflow-x: hidden implicitly computing overflow-y to auto — that was worth acting on, and commit 2b5d35a states the overflow-y: auto explicitly so a later edit can't silently remove the fallback. The remaining two (the scrollbar sitting inside the panel's 12px padding, and the earlier run's duplicate of the margin note) need no action: the Decodable Reader's scrolling grids sit inside that same padding, which is the parity this PR is aiming for.

CI (pr-automation) passed. CodeRabbit has auto_review.enabled: false on this repo, so it does not review PRs here.