feat(ui): adopt Shadcn AI Elements in the deferred workspace

[robinbraemer]

Outcome

Flect now uses Shadcn v4 as the neutral UI foundation and the official AI Elements registry for its agent experience. The protected workspace remains an Astro island on Vite: the static shell loads first, while the Preact-compatible workspace and Effect-owned runtime remain deferred until needed.

What changed

• Added the Shadcn v4/Radix component foundation and neutral light/dark tokens.
• Added official AI Elements compositions for conversation, messages, composer, reasoning, and tool activity.
• Removed superseded bespoke chat primitives and the old persistent pink accent.
• Kept agent workflows, cancellation, and resources in Effect; AI Elements owns presentation only.
• Restored the canonical protected-workspace bundle budget and emits a sorted per-island dependency report from the existing bundle gate.
• Fixed the normal Astro development origin so a real browser can create a Pi session.
• Fixed asynchronous continuity updates from overwriting an in-progress composer draft.
• Made the packaged macOS verifier locate the composer through its public accessibility role and name.
• Moved protected Settings out of the conversation rail: it uses the complete left workspace at wide sizes, and the full viewport at compact sizes.
• Simplified first run to one clear creation action; canvas-only controls stay hidden until an interface exists. The product surface no longer carries Flect branding, and a larger protected desktop drag strip is available above it.
• Replaced the global canvas-editing bar with a compact overlay trigger and a viewport-safe contextual palette attached to the selected element. Hover preview, capsule selection, Move earlier/later, resize presets, clearing, busy state, and the existing attributable agent/revision workflow remain connected.
• Added observable Light/Dark WCAG, viewport-containment, capsule-action, and full direct-edit regression coverage for the contextual overlay.
• Reworked the chat into one chronological turn timeline with compact grouped tool activity, expandable bounded details, clearer success/attention states, and a calmer composer surface.
• Moved the model, provider, and reasoning picker into a native top-layer dialog so the canvas cannot clip or cover it.

Current evidence

• Final local commit: 6698b9a.
• bun run check passed: 905 Vitest tests (1 intentional skip), type-checking, lint, Effect and product-quality gates.
• The production and diagnostic browser bundle gates pass with the protected workspace still below its decoded and gzip budgets.
• Production Chromium passed the grouped tool disclosure closed/open contract and chronological persistence through reload.
• Production Chromium passed the full Select element → target → Move later flow through the same agent and revision path.
• Production Chromium passed contextual-overlay WCAG A/AA, viewport containment, hover targeting, and screenshots in both Light and Dark appearances.
• The full production-Chromium suite passed: 91/91 flows, including settings-as-overlay, model selection, forced-colors, disposable role workspaces, local control, sharing, and the interaction/performance budgets.
• Rust formatting and 26 native tests passed; the ad-hoc signed macOS .app bundle built successfully.
• Prior packaged macOS verification passed ad-hoc signature verification, actionable setup, editable draft, sidecar-loss recovery, restored draft, native menus/controls, and one window.
• Dated baseline evidence: verification report.
• The hosted Flect quality gate passed for 6698b9a (run 31706281232).

External evidence deliberately not claimed

This host has no xctrace/Instruments, AgentOS cluster, Windows/Linux target, clean second machine, or manual VoiceOver environment. Those remain external release gates and are not represented as complete in this PR.

Review focus

• Verify AI Elements remains behind the Astro island boundary.
• Verify application workflows remain Effect-owned.
• Review the Settings workspace, starter flow, desktop drag region, and contextual selection overlay.
• Confirm neutral tokens, selected-element visibility, and contextual controls in both appearances.