Add radial canvas quick launcher

[TroopJostle]

Summary

• restore the radial RMB gesture from feat: add sticky notes and configurable radial launcher #23 on top of the unified v1.5 canvas dispatcher
• hold right-click on empty canvas, point toward an action, and release to launch it
• preserve the standard empty-canvas menu for a stationary right-click and keep region, note, terminal, Browser, plugin, and editable-surface context behavior unchanged
• let users choose and order up to eight radial actions in Agents Settings, with normalized persistence and safe defaults
• use the v1.5 shared palette tokens, UI scale, official provider/Lucide assets, and current overlay/browser ownership model

This is rebuilt from the current main, including merged PRs #27 and #28. It contains only the radial-launcher feature; unrelated local default-profile, recent-directory, and limits work is excluded.

Verification

• npm run typecheck
• npm test — 551/551 passed
• npm run build
• focused radial-launcher, context-routing, and settings-normalization tests — 51/51 passed