refactor: distill AdminOverlay complexity and inject color boldness

- Replace window.prompt() for category creation with dedicated CategoryCreationModal component
- Increase visual hierarchy: section headings slate-500 → slate-400 (bolder, more confident)
- Standardize Add User/Category button styling with consistent hover states and focus indicators
- Remove browser prompt friction; all forms now use accessible modal dialogs
- Technical, precise aesthetic: reduced visual clutter, increased contrast and decision clarity

.impeccable.md

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+# Design Context: TFM aInventory
+
+## Users
+
+**Primary Users:** Field operators AND administrative staff (hybrid usage)
+- **Field Operators:** Using on-site/in-warehouse on mobile devices for real-time scanning, inventory check-in/out, offline data collection
+- **Admin Staff:** Using on desktop for user management, category management, audit logs, system settings
+- **Context:** High-stakes environment; accuracy critical; both speed (field) and comprehension (admin) matter equally
+
+## Brand Personality
+
+**3-Word Profile:** Technical. Precise. Decisive.
+
+- **Voice:** Direct, no-nonsense, authoritative. Every element serves a clear function.
+- **Tone:** Business-focused, reliable, confident. Users trust this system with their inventory.
+- **Emotional Goal:** Confidence in accuracy; reassurance that nothing will be lost or mistyped; decisiveness in destructive actions.
+
+## Aesthetic Direction
+
+**Theme:** Dark mode (current), but bolder and more distinctive
+
+**Why Dark:** Field operators often use devices in varying lighting (warehouse basements, outdoors). Dark mode reduces eye strain and works across contexts.
+
+**Current State:** Minimal slate/blue palette. Recently removed glassmorphism (backdrop-blur). Clean but understated.
+
+**Opportunity:** Inject personality and visual weight while maintaining technical precision. The current aesthetic is correct in *restraint* but lacks *boldness*. Dark mode should feel confident and intentional, not just "safe default."
+
+**Aesthetic Inspiration:** Industrial + Technical
+- Precision instruments (clean lines, exact details)
+- Command center interfaces (high contrast, clear hierarchy)
+- Lab equipment (honest materials, no decoration)
+- NOT: Glassmorphism, gradients, glows, or decoration without function
+
+## Design Principles
+
+1. **Clarity Through Contrast:** High visual hierarchy. Field operators need to scan + act in seconds. Admins need to parse complex data quickly. Use color, size, and weight strategically.
+
+2. **Technical Precision:** Every element has intention. No decoration. Use whitespace and alignment to create rhythm, not visual effects.
+
+3. **Dual-Context Adaptation:** Field view (mobile, fast, action-focused) vs. Admin view (desktop, data-rich, decision-focused). Same visual language, different complexity.
+
+4. **Dark Mode Confidence:** Dark shouldn't mean "timid." Use bold primary colors, high-contrast text, purposeful accents. Make the dark feel deliberate.
+
+5. **Offline-First Reliability:** Suggest stability and trust. No animations that suggest pending network requests. Solid, grounded UI.
+
+---
+
+**Next Step:** Use these principles to guide `/distill` — systematically remove unnecessary complexity and inject bold, technical personality into the interface.