feat: built-in Asset Picker to replace OS file dialogs for asset selection

[esengine]

Problem

When adding frames to an Animation Clip via the Add button, the stored path is incorrect (only filename, no directory), causing "file not found" errors at runtime. Dragging assets from Content Browser works correctly.

More broadly, asset selection across the editor uses a mix of OS native file dialogs and browser file inputs, leading to:

• Inconsistent path handling (some store UUID, some store relative path, some store just filename)
• Ability to select files outside the assets/ directory, which breaks at runtime
• Inconsistent look & feel (OS dialog vs editor style)

Solution

Replace all OS file dialogs with a built-in Asset Picker — a modal window that browses only the project's assets/ directory.

What it looks like

┌─ Select Texture ─────────────────────────────────┐
│ [🔍 Search...                       ] [Grid|List] │
│┌───────────┬─────────────────────────────────────┐│
││ assets/   │  ┌─────┐ ┌─────┐ ┌─────┐ ┌─────┐  ││
││  ├ tex/   │  │     │ │     │ │     │ │     │  ││
││  ├ anim/  │  │a.png│ │b.png│ │c.png│ │d.png│  ││
││  └ fx/    │  ┌─────┐ ┌─────┐                   ││
││           │  │     │ │     │                   ││
││           │  │e.png│ │f.png│                   ││
│└───────────┴─────────────────────────────────────┘│
│                                [Cancel]  [Select]  │
└───────────────────────────────────────────────────┘

Features

• Browse assets in a familiar folder tree + grid/list view (same as Content Browser)
• Filter by asset type (images, animations, shaders, audio, etc.)
• Search across all assets
• Thumbnail previews
• Single-select and multi-select modes
• Returns correct relative path + UUID every time

Affected areas

• Animation Clip Inspector — Add frames button (fixes the original bug)
• Inspector property editors — Texture, material, shader, animation clip, timeline, audio, tilemap fields
• Material editor — Shader and texture selection
• Spine editor — Skeleton file selection
• BitmapFont Inspector — Image and .fnt file selection

Not changing

• Scene import/export dialogs (need access to files outside assets/)
• BitmapFont "Import Folder" (needs folder selection)

[esengine]

Resolved: replaced all native file dialogs with a built-in Asset Picker that browses the project assets directory, ensuring correct relative paths on all platforms.