feat(desktop): polish workbench visual system and interaction chrome

[agentfleets]

Summary

• replace the cyan/indigo-heavy treatment with restrained neutral surfaces and ChatGPT-inspired pane contrast
• normalize typography, icons, borders, glass controls, scrollbars, pane headers, titlebar behavior, and multi-tab overflow
• reorganize Read, Author, Fleet, Sessions, Terminal, Settings, hub, and status-bar controls for lower visual density
• repair popover dismissal and layering, rail visibility, native window dragging, local shell launch, HTML navigation/context menus, annotation access, and PDF selection/link overlays
• preserve the separate Visual Review workflow used for iterative macOS QA

Verification

• renderer tests: 794 passed
• Electron tests: 441 passed
• desktop production build and TypeScript check passed
• Electron build and TypeScript check passed
• desktop token lint passed
• git diff check passed
• visual QA performed iteratively in the macOS Visual Review build

Closes #538