fix: resolve widget shortcut conflict, CSP loading, and selection highlight

[yepzdk]

Summary

• Shift+C shortcut conflict: Fixed Shadow DOM focus detection so typing capital "C" in the feedback textarea no longer triggers annotation mode
• html2canvas CSP failure: Replaced <script> tag injection with fetch() + new Function() to avoid script-src CSP restrictions when loaded via browser extension
• Selected element highlight: Added a persistent blue highlight around the selected element while the feedback panel is open

Test plan

• Activate widget with Shift+C, click an element, type a message starting with capital "C" — should stay in the feedback panel
• Use Chrome extension to activate widget on a page, submit feedback with screenshot — html2canvas should load without CSP errors
• Select an element — blue highlight should remain visible while the panel is open, and disappear when closed

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