Add design and product documentation

DESIGN.md

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+# Design — FreightOps Control
+
+## Direction
+
+Dense utilitarian operations UI: stable sidebar, compact tables, status hierarchy, restrained charts, and low-noise surfaces.
+
+## System
+
+- Use semantic status tokens and tabular numerals; never encode state by color alone.
+- Prefer tables and aligned detail panels over decorative card mosaics.
+- Keep loading, empty, stale, partial, and error states explicit.
+- Motion is limited to state transitions and attention routing; reduced motion must preserve all information.
+- Mobile uses task-focused views rather than shrinking the desktop dashboard.