Port multi-row tabs setting to main

[danshapiro]

Summary

• Port the feat: add multi-row tabs setting (per-client, off by default) #335 multi-row tabs setting from the stale dev-stack PR Stabilize dev green and canonical agent recovery #342 onto current origin/main.
• Add the per-client multirowTabs local pane setting, Settings > Panes toggle, local browser-preferences persistence, wrapped TabBar layout, and docs mock CSS.
• Carry focused unit and e2e test coverage for the setting and layout behavior.

Source and scope

• Source payload: feat: add multi-row tabs setting (per-client, off by default) #335 commit 00a54902bb88c25c04dc7e49e2200188377e9f31 (feat: add multi-row tabs setting (per-client, off by default)).
• This branch intentionally does not port Replace tabs registry with compact device snapshots #331 compact tabs-registry content; Port compact tabs-registry device snapshots to main #355 covers that separately.
• While resolving the TabBar.tsx conflict, this port preserved current main sidebar reopen placement instead of pulling in the unrelated dev parent change that had moved it outside the tab scroller.
• Added one main-scoped browser-preferences persistence assertion for multirowTabs.

Verification

• npm run test:vitest -- test/unit/shared/settings.test.ts test/unit/client/store/browserPreferencesPersistence.test.ts test/unit/client/components/TabBar.multirow.test.tsx --run passed: 3 files, 43 tests.
• npm run typecheck:client passed.
• npm run build:client passed, with existing Vite dynamic-import and chunk-size warnings.
• npm run lint passed with 14 existing warnings and 0 errors.
• npm run typecheck:server failed before this branch's touched code due to existing origin/main parse errors in server/mcp/freshell-tool.ts lines 73 and 472.
• npm run build:server failed on the same existing parse errors.
• npm run test:e2e:chromium -- test/e2e-browser/specs/multirow-tabs.spec.ts was blocked in global setup because it runs npm run build:client && npm run build:server, and build:server fails on the same existing parse errors.

Supersedes

This PR supersedes the #335 multi-row tabs setting payload currently embedded in stale #342.