fix: remove color palette from unsupported diagrams

[priyanshu88064]

What changes were made

• Diagram-Aware Node Detection: Updated [getNodeIdFromLine] to use regular expression based on the diagramType.
• Support-Based Palette UI: Introduced SUPPORTED_COLOR_DIAGRAMS to restrict the color palette UI to diagram types that support standard style NODE fill:#color syntax.
• Append-Only Style Management: Updated upsertNodeStyleLine to append new style directives to the end of the file instead of inserting them immediately after node definitions.

Why the changes were needed

• Fix ER Diagram Bug: Fixed an issue where the relationship separator (:) was misidentified as a node ID in ER diagrams.
• Prevent Invalid Syntax: Diagram types like Sequence Diagrams do not support standard style blocks hiding the palette for these types.
• Preserve Block Structures: For diagrams with block-based definitions (like class Auth { ... }), inserting styles mid-file could break the block structure. Appending to the end of the file ensures the Mermaid syntax remains valid.

[ZEROCONNECT]

@priyanshu88064 Please update getDiagramType so it matches Mermaid’s preprocessing before detecting the diagram type. In particular, strip YAML frontmatter and Mermaid init directives/comments before returning the first token. Otherwise valid diagrams with frontmatter render correctly but lose the color palette.