fix(web): scroll runtime bar instead of spilling the shell

[jonathanung]

Summary

• Expanding Runtime… now scrolls the config bar (overflow-x: auto) instead of growing .app-shell / main.
• .runtime-stack gets min-width: 0; secondary fields keep min-content so the bar is the scrollport.
• Regression tests at 900px and 360px: shell/main scrollWidth stays within clientWidth.

Issue

Fixes #1246

Tier

B

Verification

• test -z "$(gofmt -l .)"
• make web-check
• make test && make vet && make build
• smoke: skipped (web CSS/layout only; no cmd/engine/tui/session/auth)

Note: TestCollectForwardedEnv failed once locally when host AWS_PAGER= matched AWS_*; 3/3 pass with env -u AWS_PAGER. Unrelated env flake.

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[jonathanung]

Review pass 1 (head c492a009438947a4766dcf5ae06152b37f787f8f) — clean

Verdict: approve (internal; COMMENT only)

0 blocking, 0 should-fix. Merge checklist may proceed for this SHA only.

Reviewed gh pr diff 1248 plus surrounding runtime CSS (.runtime-stack / .runtime / .runtime-more / .runtime-secondary) and the existing runtime op tests.

• Expanding Runtime… stays inside .runtime (overflow-x: auto); stack is min-width: 0 so main / .app-shell do not grow.
• Secondary fields use min-content / flex-shrink: 0 so the bar is the scrollport instead of leaking visible overflow.
• CSS contract in styles.layout.test.ts; App test expands at 900px and 360px and checks containment. jsdom cannot measure real scrollWidth; metrics are modeled from those rules (acceptable for unit gate; e2e already has page-level overflow checks).

Nits (deferred)

• App.test.tsx duplicates the CSS rule() helper already in styles.layout.test.ts.
• Modeled scrollWidth will not catch a future override that keeps the same declarations but still expands the shell.