[Subtask] Improve and unify node/edge attribute editing in the UI

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Overview

Improve the existing UI for editing node and edge attributes so that attribute management becomes a first-class workflow rather than an isolated or partial capability.

This subtask focuses on editing, discoverability, consistency, and persistence.

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Motivation

SocNetV already has parts of an attribute/metadata system, but users need a clearer and more complete way to:

• inspect attributes
• add new attributes
• edit existing attributes
• remove attributes

for both nodes and edges.

This is the foundation for all structured data workflows in #223.

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Scope

Node attributes

• inspect existing attributes
• add / edit / remove attributes

Edge attributes

• inspect existing attributes
• add / edit / remove attributes

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UI Goals

• consistent editing workflow for nodes and edges
• predictable save/load behavior
• clear distinction between:
  • built-in properties
  • custom attributes / metadata

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Possible UI Directions

• improve existing node edit dialog
• add edge attribute editor
• show attributes in:
  • side panel
  • properties dialog
  • optional tooltip / inspector later

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Implementation Notes

• Reuse existing attribute infrastructure:
  • [Feature Request] Custom Attributes for Nodes and Edges (Incl. Dates) #96 Attribute system
  • [Core] Node metadata system (import, storage, and usage) #130 Node metadata
• Preserve current behavior where already implemented
• Avoid duplicating logic across multiple dialogs

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Edge Cases

• empty attribute values
• duplicate keys
• type ambiguity (string vs numeric)
• save/load consistency across formats

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Relation to Feature

Part of:
#223 Graph data editing and structured data workflows

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Implementation Plan

What already exists (no changes needed)

• Node attribute storage in GraphVertex (QHash<QString,QString>)
• Graph-level node attribute API: vertexCustomAttributes(), vertexCustomAttributesSet(), graphHasVertexCustomAttributes()
• Node Properties dialog with full add/remove/edit via QTableWidget
• GraphML node attribute roundtrip (export + import)

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Phase A — Node API hardening

Add two thin wrappers to Graph (over the existing bulk API):

• vertexCustomAttributeSet(v, key, value) — set one key without replacing all attributes
• vertexCustomAttributeRemove(v, key) — remove one key

Needed by the filter system (#217) and avoids callers doing get→modify→set themselves.

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Phase B — Edge custom attribute storage

Add to GraphVertex, consistent with existing m_outLinkColors / m_outEdgeLabels pattern:

QHash<int, QHash<QString,QString>>  m_outEdgeCustomAttributes;  // keyed by target vertex

Add GraphVertex methods: setOutEdgeCustomAttributes(v2, hash), outEdgeCustomAttributes(v2).

Add to Graph:

• edgeCustomAttributesSet(v1, v2, attrs)
• edgeCustomAttributes(v1, v2)
• graphHasEdgeCustomAttributes() → QStringList of all unique keys across all edges

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Phase C — Edge Properties dialog

edgeeditdialog.ui stub exists but has no .cpp/.h. Implement it using the dialognodeedit pattern:

• QTableWidget for custom attributes (key/value rows, add/remove)
• Emit signal with final attrs on accept
• Wire into MW: slotEditEdgeProperties() slot, connected from edge right-click context menu

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Phase D — GraphML roundtrip for edge attributes

Symmetric to the existing node attribute export/import in graph_io.cpp:

• Export: write <key for="edge"> definitions + per-edge <data> tags
• Import: parse edge <data> tags and call edgeCustomAttributesSet()

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Phase E — Foundation for #217

Add Graph::vertexFilterByAttribute(key, value) using the existing snapshot/restore system (m_visibilityHistory). Hides nodes where the attribute key is absent or value does not match. This is the first step of #217 — done here to validate the data layer is correct end-to-end.

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Order

A → B → C → D → E. Each phase is independently testable.
A and B are pure model work; C is UI; D validates the model via roundtrip; E closes the loop to #217.

Out of scope for #224

• Edge attribute filtering (that is [Subtask] Filter nodes and edges by attributes (metadata-based filtering) #217 proper)
• Attribute-based node coloring/sizing ([Feature Request] Custom Attributes for Nodes and Edges (Incl. Dates) #96 followup)
• Temporal attributes ([Subtask] Support temporal attributes and time-based filtering (timeline view) #222)