ui: reshape Iris around a professional multi-pane shell

[avabbbb]

What changed

This is a first-pass shell refactor inspired by the supplied professional-app wireframe.

• adds a persistent left rail for Canvas / Table / Agent
• adds a collapsible project sidebar beside the rail
• keeps the existing main production surface intact
• keeps the current docked Assistant / Context / History right drawer intact
• adds a compact bottom status strip
• preserves the existing top bar and existing workflow/business logic
• responsive behavior hides the project sidebar on narrower windows

Why

The goal is to move Iris away from a web-app/dashboard silhouette and toward the spatial grammar used by professional creative tools: rail → browser/sidebar → main stage → contextual inspector → status bar.

This PR intentionally changes the shell only. It does not duplicate Workflow authority, add a second timeline/canvas, or rewrite Agent behavior.

Follow-up after visual review

If this structure feels right, the next slice should reduce duplication in the current top bar and make the left project sidebar context-aware (Projects / Assets / Host context) rather than adding more controls.

Verification requested

• desktop widths: 1440 / 1280 / 1024
• narrow layout below 880px
• Canvas / Table / Agent switching
• project switching and project creation
• right drawer docked + overlay states
• light / dark themes