feat: dynamic UI workspace, collapsible activity rail, and sticker studio

[spelech]

Summary

Transforms the UI shell from rigid stock Avalonia horizontal tabs into a modern dynamic workspace with an Activity Rail, a Dynamic Stage Container, and an integrated Titlebar Telemetry Ribbon. Introduces the end-to-end Sticker Studio creative workflow.

Key Changes

1. ActivityRailControl:
   • Collapsible left rail: 56px icon-only mode with tooltips, expandable to 200px drawer with labels.
   • Primary domains: ⚡ Studio, 📦 Models, 💻 Hardware Fit, ⚙️ Settings.
   • Utilities: 📖 Documentation, 🤖 AI Assistant, and collapse/expand toggle.
2. TelemetryRibbonControl:
   • Compact 34px status ribbon replacing the bulky 90px header.
   • Embedded directly into Desktop custom titlebar and Web top header.
   • Live service status dots (Ollama, Forge SD, ComfyUI) and VRAM usage badge.
3. StickerStudioControl & ImageDropZoneControl:
   • Input Deck (380px): Native drag-and-drop reference image drop zone with file picker fallback, 6 art style preset chips (Die-Cut Vinyl, Holographic, Chibi Anime, 80s Retro, Pop Art, Watercolor), prompt customization, die-cut white border slider (0–24px), and auto-cutout subject toggle.
   • Output Canvas: Alpha-transparency checkerboard preview, 4-stage pipeline progress tracker, and quick-export action pills (Copy PNG with alpha, Save File).
4. DynamicStageContainerControl:
   • Dynamically switches stages based on selected domain while preserving backward compatibility with tab routing, companion popouts (Desktop), and in-canvas slide-out drawers (Web).

Verification

• Avalonia Layout Audits: 10/10 tests passing across 1440x900 (Desktop), 1024x768 (Tablet), and 1280x800 with 320px Web Drawer active, with 0 boundary overflows, 0 sibling collisions, and 0 text clipping violations.
• Solution Test Suite: All 781 tests passing.
• Web Tooling:
  pm run lint and
  px tsc --noEmit passing with 0 errors.