[#727] feat: implement manual range authority and refined segmentation UI

[wayangalihpratama]

Logical Model (Manual Range Authority)

• Implemented "Manual Range Authority": User-defined Min and Max values are strictly respected during recalculation and final segment generation.
• Bi-directional Cascading: Implemented helpers for numerical range editing (Segment N Max updates Segment N+1 Min and vice-versa) using a robust 0.01 precision offset.
• Adjust Logic: Refined the "Adjust" button to send both boundaries simultaneously, preventing range reversion bugs.

UI/UX Improvements

• Layout Simplification: Adjusted grid spans to 12/12 in Step 2 and added "Min/Max" input prefixes for better clarity.
• Naming Enforcement: Enforced manual, unique naming for numerical segments with real-time frontend validation and empty-by-default inputs.
• Dynamic Display: Decoupled "Segment range" display from live form fields and made it optional via a new toggle.
• Prioritization: Swapped button order to prioritize manual segment addition.
• Enhanced Labels: Updated variable selection labels to be more descriptive.

Backend Fixes & Stability

• UnboundLocalError: Resolved a scoping bug in backend processing during numerical segment value generation.
• Missing Data Fix: Implemented inclusive lower bounds (>=) for starting segments to ensure all farmers are captured in the analysis.
• React Crash Fix: Implemented robust error stringification for Pydantic/FastAPI validation errors and sanitized payloads to prevent frontend crashes.

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Close #727

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