feat(demo): exercise AI Assistant, dock, corners and custom icons

[sidgaikwad]

Fixes #40.

The demo is the main thing people try before adopting the component, but several documented options were never wired into it — including the AI Assistant, the README's headline feature.

Added

Option	How
projectId + features.ai	AI Assistant toggle, gated on VITE_UNLAYER_PROJECT_ID. When unset, the controls are replaced by a pointer to the new demo/.env.example, so the demo still works with no Unlayer account.
features.imageEditor.dock	Left/Right tool-rail selector.
tools.corners	Was missing from TOOL_NAMES entirely.
custom tool icon	Raw <svg> markup on the Text tool.

Fixed: the snapshot lied about its blob

setSaved({ dataUrl, blob: new Blob() });   // 0 bytes

The Download link worked (it uses dataUrl), but the object claimed to be an ImageEditorSaveResult while fabricating blob — misleading in the file people read to learn the API. It now builds the real blob:

Snapshot taken via getImage() (1998122 bytes)

⚠️ A bug I found doing this — #64

The README's own Font Awesome icon example does not work. I started with icon: 'fa-font', saw no change, and checked properly against the live embed:

icon value	result
'fa-crop-simple' (the README's example)	❌ silently ignored, default icon stays
'fa-star'	❌ silently ignored
raw '<svg …>'	✅ works

Filed as #64. This PR uses the raw-<svg> form, so the demo ships an example that actually does something rather than one that silently no-ops.

Verification

Driven against the live CDN embed at 1280×800:

• new sidebar sections present: Layout (remounts editor), AI Assistant
• corners present in the tool toggles
• dock switch moves the rail: left: 968 → left: 0 in a 1040px editor
• custom SVG icon rendered in the rail
• snapshot produces a real 1,998,122-byte blob
• AI hint shown (no projectId configured in my environment)

tsc --noEmit, vite build, root tests and Prettier all clean.

Left out deliberately

translations, offline/licenseUrl/env and scriptUrl. Each needs real scaffolding (a translations fixture, a license file, an alternate CDN) that would make this PR much harder to review, and none is as load-bearing as the AI Assistant gap. Happy to follow up.

Conflict note

Touches demo/src/App.tsx, Sidebar.tsx and styles.css, so it will conflict with #59 (demo responsive/theme).

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[sidgaikwad]

Heads-up on an overlap I missed before opening this: #30 by @brenopolanski already adds a dock control to the demo, and predates this PR by two days.

The dock portion here duplicates that work. Suggested resolution: merge #30 first and I will rebase this to drop the dock section, keeping the AI Assistant gating, corners, the custom icon and the blob fix — or if you would rather take this one whole, #30 should be closed in its favour. Happy either way; #30 was there first, so my preference is the former.