feat(upload): support pasting clipboard files

[JustinShetty]

Adds a paste event listener which infers files types for files based on their MIME type, or takes the existing name if it has one already.

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Summary by cubic

Adds support for pasting files from the clipboard in the upload drop zone.

• Ignores pastes in editable fields (inputs, textareas, selects, and contenteditable) so text pasting is unaffected.
• Keeps the pasted file's name when it includes an extension; otherwise generates a timestamped name from its MIME type, falling back to .bin.
• Renders file names as plain text to prevent HTML injection.
• Updates the drop zone hint and loads the MIME mapping from WEBROOT.

^{Written for commit b79d914. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 2 files

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[cubic-dev-ai]

All reported issues were addressed across 1 file (changes from recent commits).

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[cubic-dev-ai]

All reported issues were addressed across 3 files (changes from recent commits).

<sub>Tip: Review your code locally with the cubic CLI to iterate faster.

Re-trigger cubic</sub>

[cubic-dev-ai]

All reported issues were addressed across 3 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>