Closes #7876

After reverting DOMPurify-based sanitization (#7886) because it stripped
`<foreignObject>` (needed for text) and `<style>` (needed for fonts),
the SDK had no sanitization of pasted/dropped SVGs. This PR adds a
custom allowlist-based sanitizer (~643 lines, <3KB gzip) that blocks
attack vectors while preserving tldraw's own SVG output. The
tag/attribute allowlists and URI sanitization approach are derived from
[DOMPurify](https://github.com/cure53/DOMPurify) (MIT license).

### What it does

**Root element validation:** Rejects documents whose root is not
`<svg>`.

**Mode-switching sanitizer:** Walks the SVG DOM tree, switching between
SVG and HTML sanitization modes at `<foreignObject>` boundaries. This
preserves tldraw's text rendering (foreignObject with HTML) while
applying the correct allowlist in each context.

**Context-dependent URI sanitization:**
- `<image>` / `<feImage>`: raster `data:` only (no `svg+xml` — could
embed unsanitized SVG)
- `<use>`: fragment refs (`#id`) only
- `<a>`: `http:`, `https:`, `mailto:` only

**CSS sanitization:** Strips `@import`, `expression()`, `-moz-binding`,
`behavior:`, and external `url()` references. Preserves `data:`
font/image URLs and `url(#id)` fragment references (needed for
gradients/filters). Handles multiline `url()` values (dotAll flag) and
CSS escape decoding with bounds checks.

**URL-bearing presentation attributes:** Sanitizes `url()` values in
`fill`, `filter`, `clip-path`, `mask`, `stroke`, `marker-*`, `cursor` —
not just `href`/`xlink:href`. Case-insensitive matching. External
`url()` references in these attributes are stripped while `url(#id)`
internal refs are preserved.

**Animation safety:** `<animate>`, `<set>`, `<animateColor>`,
`<animateTransform>` elements targeting `href`, `xlink:href`, or `on*`
via `attributeName` are removed (prevents runtime injection of
`javascript:` URIs that bypass static sanitization).

**Event handler stripping:** All `on*` attributes stripped after
invisible whitespace normalization — catches current and future handlers
without maintaining a blocklist.

**Integration:** Applied at all 4 SVG entry points via dynamic import
(keeps sanitizer out of main bundle). `maybeSanitizeSvgFile` has
try-catch so one bad file doesn't abort a batch. Failed SVG sanitization
shows a toast in multi-file drops. SVG text paste returns silently on
failure. File-replace returns silently on failure (called without
await).

### Change type

- [x] `improvement`

### Test plan

1. Paste tldraw-generated SVG (copy as SVG) — text, fonts, foreignObject
preserved
2. Paste malicious SVGs (script, onerror, onload, javascript: href) —
all blocked
3. Paste SVG with animation targeting href — animation removed
4. Paste SVG with external url() in fill/filter/mask attributes —
stripped
5. Drop multiple files including a malicious SVG — toast shown, other
files still processed

- [x] Unit tests (76 tests covering attack vectors, preservation, and
round-trip with all shape types)

### API changes

- Added `sanitizeSvg(svgText: string): string` — public export for SDK
users building custom handlers

### Release notes

- Add SVG sanitization on paste and file drop to prevent XSS and data
exfiltration