* refactor(app): isolate system clipboard adapters
- Split browser and Tauri clipboard behavior into focused adapters\n- Inject browser clipboard capabilities into unit-testable operations\n- Remove navigator and document mutation from headless clipboard tests
* refactor(app): delegate browser clipboard fallbacks
- Use copy-to-clipboard for modern rich MIME writes and execCommand fallback\n- Keep OpenPencil-specific HTML and plain-text payload construction at the adapter boundary\n- Remove hand-rolled browser capability and selection handling
* test(clipboard): verify rich browser menu round-trip
- Exercise copy from the browser Edit menu under a user gesture\n- Verify text/html and text/plain ClipboardItem formats\n- Paste the system clipboard payload back into the canvas
* refactor(clipboard): reduce adapter surface
- Expose only command dispatch and the injectable system clipboard contract\n- Keep browser and Tauri copy/paste operations private to their adapters\n- Rename the in-memory DataTransfer fallback and share design HTML recognition
* fix(clipboard): harden format and fallback handling
- Require complete OpenPencil or Figma clipboard markers\n- Await browser writes so adapter failures resolve false\n- Write plain-only Tauri payloads as text and reject unrelated clipboard text
* fix(clipboard): reject unrelated current browser data
* fix(clipboard): bind fallbacks to copied selection
- Match cached rich HTML to the current Tauri plain-text fallback\n- Preserve nodes when selection changes during an asynchronous cut\n- Reject unrelated current browser HTML before consulting memory
* fix(clipboard): reuse the shared memory payload type
* fix(clipboard): scan design markers linearly
* fix(clipboard): distinguish unavailable and empty reads
* style(clipboard): use includes for marker closure
* test(clipboard): avoid wall-clock marker assertions
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