fix(ui): correct workspace menu width and tooltip stacking

[kev1n77]

Summary

• Fix the oversized workspace menu by arranging recent projects in a vertical flex layout.
• Place shared tooltips above popovers and nested menus.
• Replace hard-coded menu z-index values with shared layer tokens across navigation, session actions, device switching, and tab overflow menus.
• Add regression coverage and regenerate the Data Migrator token stylesheet.

Type and Areas

Type: Bug fix / UI/UX

Areas: Web UI, shared design tokens, navigation, session overview, documentation, tests.

Motivation / Impact

The workspace menu incorrectly accumulated the intrinsic widths of recent-project rows, causing it to expand to its maximum width.

Tooltips could also render behind the panels containing their triggers. This affected sub-agent session actions, background command actions, model and context pickers, and truncated labels in project, device, and tab menus.

The changes restore content-sized menus and keep tooltips above their associated panels while preserving backdrop and submenu ordering.

Verification

The following checks passed during implementation:

• pnpm run design-system:check
• pnpm run check:web
• pnpm --dir src/web-ui run test:run src/infrastructure/appearance/runtime/AppearanceOverlayHost.test.ts src/flow_chat/components/ModelSelectorPortalLayer.test.ts — 14 tests passed.
• git diff --check

Final desktop visual acceptance and remote-scenario integration testing have not been performed.

Reviewer Notes

• Ordinary menus use layer 360, nested menus use 361, and tooltips use 370.
• Toasts, notifications, and context menus retain higher priority.
• No persisted data, protocol, or user-facing string changes.
• The Data Migrator stylesheet was regenerated from the shared token source.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.