Fix squished diff toolbar in narrow panels

[ymichael]

Summary

• let the diff selector, large-change tally, and actions wrap from the toolbar container's available width
• preserve a readable selector basis and non-shrinking stats/actions without adding viewport or resize state
• align wrapped stats with the selector label inset and cover the intrinsic layout policy plus control operability

Root cause

The toolbar switched layouts only when the entire app entered its compact drawer. A resizable right panel on a 900px-wide page could therefore remain in the inline layout at just 257px wide, shrinking the selector to 0px and truncating the large-change tally. The revised flex policy wraps intrinsically from the toolbar's own width.

Validation

• pnpm exec turbo run test --filter=@bb/app --force -- src/components/secondary-panel/ThreadSecondaryPanel.test.ts src/components/secondary-panel/ThreadSecondaryPanel.collapseControl.test.tsx (17 tests)
• pnpm exec turbo run typecheck --filter=@bb/app
• pnpm exec turbo run lint --filter=@bb/app --force (no errors)
• dev-browser mobile QA at 412x915 with 351 files, +38,872 / -22,464; verified readable controls, responsive menu, file expand/collapse, and no page-level overflow
• dev-browser narrow desktop-panel QA at a 900x915 viewport and 257px toolbar; verified the selector stays readable, the full tally remains visible, controls wrap, and no page-level overflow appears
• dev-browser wide desktop sanity check at 1440x1000; verified all controls remain on one row

Closes #1652

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AGENT GENERATED: by GPT-5.6

[bb-slop-cop]

🚨 SLOP COP 🚨 · review

I am SlopCop. I am reviewing this pull request for security, code quality, performance, architecture, and compact-screen behavior.

[bb-slop-cop]

🚨 SLOP COP 🚨 · review

Plain-English summary

This change lets the Git diff toolbar move controls onto more rows when the panel becomes narrow. Wide panels keep one row.

Findings

I found no actionable security, quality, architecture, duplication, or performance problems.

The final commit uses the toolbar width directly. It does not copy viewport rules into its callers.

Validation

• Three agents reviewed security, code quality, architecture, duplication, and performance.
• A GPT-5.6 final review gate returned APPROVE.
• The focused toolbar test passed.
• The app type check passed.
• The diff check passed.
• Browser checks passed at 320 and 760 pixels. All controls stayed inside the viewport.
• The full app lint did not finish within eight minutes. It produced no result.

The author changed the head during review. I repeated the focused checks against d6c691958e8bbc907b7753d3a1fff74a0f732be4.

[ymichael]

CI follow-up: workflow run 32077636826 was allowed to finish, then its failed integration job was rerun. Attempt 1 failed in fake/smoke/custom-models.test.ts because buildPluginHost could not read plugins/provider-acp/dist/.host-stage-*/host.js. Attempt 2 failed in the same plugin-host staging area: the custom-model test passed, while other integration tests hit missing staged provider-acp/host.meta.json, missing staged provider-claude-code/host.js, and an ENOTEMPTY cleanup error (rerun job).

This is unrelated integration infrastructure flakiness: this PR changes only GitDiffToolbar.tsx and its component test; app tests and the build/typecheck/lint check passed. The same buildPluginHost .host-stage-* failures also occurred on current main run 32076731685 and unrelated PR run 32077823686. No provider/plugin/integration code was changed here.

[ymichael]

Final CI follow-up: current head f42338b6e passed the complete CI workflow 32078805247, including the integration job that had flaked in the earlier run. This confirms the earlier buildPluginHost .host-stage-* failures were unrelated infrastructure flakiness.

[ymichael]

CI note for current head 4f330e0ab: workflow 32079692491 passed checks/build/typecheck/lint, app tests, package tests, server tests, and both package-smoke jobs. Its integration job hit the known unrelated buildPluginHost staging race twice: attempt 1 failed reading provider-acp/dist/.host-stage-*/host.js; the failed-job rerun changed which lifecycle test failed and then failed renaming another staged provider-acp/host.js (rerun job). Each attempt otherwise passed 54/55 integration tests.

This PR still changes only the diff toolbar and its component test. The immediately preceding head passed the complete integration job in run 32078805247, further confirming nondeterministic infrastructure flakiness. No plugin-host or integration code was changed here.

[ymichael]

Current head 933be8d99 is fully green in CI run https://github.com/get-bb/bb/actions/runs/32080653729. The integration suite passed on this run, superseding the prior plugins/provider-acp/dist/.host-stage-* ENOENT failures and confirming that failure was nondeterministic infrastructure flakiness unrelated to the diff-toolbar files.