fix: keep modals above the global findings banner

[damusix]

When a model has global findings, the fixed banner (z-index 200) sat above modals (100), so on short viewports it covered a tall modal's close button. CI on main now fails test-graph-bundle-mode on this (re-ran main's last green run on the current runner: Bun 1.4.2, Linux).

• Modal backdrops move to 210 (stacked 220), still below the flow edge tooltip (9999).
• The JSON value dialog is portaled to document.body: the Dictionary view is its own stacking context and trapped the dialog under the banner.

test-modal-above-banner.ts covers the entity modal, a Dictionary JSON dialog, and stacked backdrop clicks; it fails without either change. Unblocks #49.

[damusix]

Folded into #49 (one task, one PR); the banner fix is what unblocks #49's CI.