fix: adaptive terminal-grid scrolling based on usable row height

[web3dev1337]

Summary

• Replaces the magic ">16 visible pairs" scroll threshold in updateTerminalGrid with a height-based rule: the grid becomes scrollable exactly when squeezing all rows into the viewport would push a terminal-pair row below a usable minimum height (200px — same floor the scrollable mode's grid-auto-rows: minmax(200px, auto) already uses).
• Re-evaluates on window resize (debounced handler), so the decision adapts to the actual window instead of a fixed count.
• Fixes both sides of the old cliff: ≤16 pairs on a short window no longer crush into unreadable slivers, and >16 pairs on a tall monitor no longer scroll unnecessarily.

Test plan

• Workspace with 17 worktree pairs at ~900px height: grid scrolls with ≥200px rows (unchanged from before).
• Same workspace on a taller window: scroll disappears once rows fit at ≥200px.
• Workspace with ~10 pairs in a short window: grid now scrolls instead of crushing tiles.
• node --check client/app.js passes.

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