feat: add LSTM layer with purple node color

[Sickwoman]

• Add customlstm node type (units + returnSequences params)
• Purple color rgb(139,92,246) distinct from all existing nodes
• LSTMNode component with configured/unconfigured display
• NodePropertiesPanel with units (max 10000) + returnSequences
• canSaveModel refactored to validation map pattern
• Strong validation: NaN, zero, negative units all blocked
• Backend _build_layer handles customlstm with safe int parsing
• Backend _build_layer handles customdropout
• 5 backend tests in test_lstm_layer.py
• 10 frontend tests covering edge cases

Description

Brief summary of the changes. Reference any related issues.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

How Has This Been Tested?

Describe the tests you ran to verify your changes.

• Existing tests pass
• New tests added
• Manual testing

Checklist

• My code follows the project's style guidelines
• I have performed a self-review
• I have added/updated documentation as needed
• My changes generate no new warnings
• Tests pass locally

[Sickwoman]

By mistake i didn't rebased it and clicked update branch. Now what should i do @ivantha