fix: cleaner Parent Node icon + rename the mind-map insert tile (#255)

[vibhavkatre]

What

Two polish fixes to the Mind Map entry in the editor's (+) insert catalog:

1. New icon. Replaced the unclean glyph (a central node with a branch to each side) with a single-line, lucide.dev-styled glyph: one parent node with three curved connectors branching off to the right — matching the Whimsical parent-node reference.
2. Renamed the tile from "Mind map" to "Parent Node" (it drops the root/parent node of a mind map). The catalog search still finds it under "mind map" as well as "parent node"; the "MIND MAP" section heading is unchanged.

Scope

Cosmetic + label only — no behavior change. The glyph is used in exactly one place (the (+) catalog tile), so the change is fully contained.

Tests

parentNodeTile.test.js (source-inspection, house pattern) pins:

• the tile label is "Parent Node",
• the catalog search matches both "parent" and "mind",
• the glyph is one node + three connectors with no end dots,
• and — guarding Inserting a flowchart/mind map places it far below the viewport; want click-to-place #75/feat: click-to-place mind-map / flowchart inserts (#75) #200 — that the tile arms click-to-place rather than dropping the node directly.

Full suite green (708 tests) + build clean locally.

Closes #255

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