feat(web): unify appearance and session surfaces

[GCWing]

Summary

• add the monochrome appearance, structural chrome/config-page token slices, and refreshed light appearance projections
• centralize floating/dialog surface chrome and FlowChat typography roles across navigation, settings, chat, notifications, and tool surfaces
• expand the FlowChat session overview with Agent tree, background command, pull-request, and right-panel controls while preserving assistant-owned Harness policy
• extend native Desktop E2E coverage assets for appearance, navigation, popup consistency, settings, and subagent identity

Verification

• focused Web Vitest: 11 files, 101 tests passed
• pnpm run theme:color-audit:all
• pnpm run i18n:audit
• pnpm run theme:visual-contract
• pnpm run appearance:contract-audit
• pnpm run motion:audit
• pnpm run check:repo-hygiene
• git diff --check

Not run / known boundary

• Native E2E was not run, as requested. No native Desktop visual/interaction validation is claimed.
• Remote Workspace, Remote Control, Peer Device Mode, and Detached Dispatch were not exercised.
• pnpm run type-check:web is currently blocked outside this PR's diff: src/infrastructure/api/adapters/websocket-adapter.ts imports GitTrustReport, but @/generated/api does not export it.