Support nested bins (parent/child hierarchy)

[akifbayram]

Problem

Users with complex storage setups (e.g., a toolbox with drawers, a filing cabinet with folders) can't represent containment relationships between bins. Everything is flat, making it hard to model real-world storage hierarchies.

Proposed Solution

Allow bins to have an optional parent bin, creating a tree structure.

Data Model

• Add parent_id TEXT REFERENCES bins(id) ON DELETE SET NULL to the bins table
• Enforce max depth of 3 levels to keep UI manageable (e.g., Cabinet → Drawer → Section)
• A bin's location_id must match its parent's location_id

API Changes

• POST /api/bins and PUT /api/bins/:id accept optional parent_id
• GET /api/bins returns parent_id and parent_name fields
• GET /api/bins/:id returns children: Bin[] array
• Deleting a parent bin nullifies parent_id on children (they become top-level)

UI/UX

Bin List Page

• Default view remains a flat grid — nested bins show a subtle "contains X bins" badge
• Add a "Tree View" toggle (icon button next to sort) that switches to an indented, collapsible list layout
• Tree view: parent bins are expandable rows; clicking expands to show children inline

Bin Detail Page

• "Parent Bin" selector field (searchable dropdown of bins in the same location, excluding self and descendants to prevent cycles)
• "Contains" section listing child bins as clickable cards
• Breadcrumb trail at top: Location > Parent > Current Bin

Bin Create Dialog

• Optional "Parent Bin" field — pre-filled if creating from within a parent bin's detail page

Scope

• No drag-and-drop reparenting in v1 — use the edit form
• Search results remain flat (but show parent name as context)
• Export/import preserves hierarchy via parent_id references