FE-1541: Add a generic ResizeHandle and adopt it across the panels

[kube]

🌟 What is the purpose of this PR?

One shared edge-resize implementation for the package. Adds the generic ResizeHandle primitive (a drag handle around the existing useResizeDrag, with min/max clamping and two visual appearances) and moves the package's two hand-rolled copies onto it. Bottom of the Notebook view stack, but independent of it: everything here is existing-UI refactoring plus one new component.

🔗 Related links

• FE-1541 (internal)

🔍 What does this change?

• ui/resize/resize-handle.tsx: the primitive — edge (top/bottom/left/right), controlled size with min/max clamping, roving-tab-order-friendly (tabIndex={-1}), and an appearance prop: "hidden" tints the strip on hover, "line" shows a centred pill; both turn blue while resizing. It exposes data-resizing/data-resize-edge so ancestors can style themselves against an active drag with :has().
• GlassPanel drops its inline handle (styles, positioning switch, drag wiring) and renders the primitive; its dragged-edge border highlight now derives from the handle's data attributes via :has(). The LeftSideBar, PropertiesPanel, and BottomPanel ride this unchanged.
• The AI assistant panel drops its hand-rolled pointer-drag path for the primitive with the line appearance, keeping its hover-pill affordance. Its shell already clamps to the viewport in CSS, so a static maxSize replaces the per-drag window-width computation.
• A layer declaration for ui/resize in the architecture docs.

Net −162 lines: three implementations become one.

Pre-Merge Checklist 🚀

🚢 Has this modified a publishable library?

This PR:

• does not modify any publishable blocks or libraries, or modifications do not need publishing
  • behaviour-preserving refactor of unreleased-alongside code; the Notebook stack above carries the release changeset

📜 Does this require a change to the docs?

The changes in this PR:

• are internal and do not require a docs change

🕸️ Does this require a change to the Turbo Graph?

The changes in this PR:

• do not affect the execution graph

🛡 What tests cover this?

No unit tests — the primitive is DOM-drag behaviour. Verified by driving the browser: all three GlassPanel-backed panels resize with the dragged-edge highlight reacting, and the AI panel resizes with exact pixel deltas through its new handle.

❓ How to test this?

1. yarn workspace @hashintel/petrinaut storybook
2. In any editor story, drag the sidebar, properties, and bottom panel edges — the panel border should highlight while dragging.
3. In the WithAiAssistant story, open the assistant and drag its left edge — a pill line shows on hover and while resizing.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
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petrinaut-docs	Ready	Preview	Aug 29, 2026 2:18pm

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[cursor]

PR Summary

Low Risk
Behavior-preserving UI refactor in unreleased petrinaut panels; no auth, data, or API surface changes.

Overview
Introduces a shared ResizeHandle primitive (built on useResizeDrag) with edge selection, min/max clamping, hidden vs line appearances, and data-resizing / data-resize-edge for parent styling.

GlassPanel drops its inline handle, drag wiring, and CVA-based border variants; it renders ResizeHandle and highlights the dragged edge via :has() on those data attributes. Sidebar, properties, and bottom panels inherit this path unchanged.

The AI assistant replaces a custom pointer-drag resize with ResizeHandle (appearance="line") on a zero-width anchor so the handle sits on the card’s left border; width limits use static 320–720 instead of per-drag viewport math (the shell already caps width in CSS).

Adds a ui.resize layer note on use-resize-drag. Net consolidation of three resize implementations into one.

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