feat(desktop): support all local path uploads from prompt input

[anduimagui]

Issue for this PR

Related to #18143

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This teaches the desktop prompt flow to treat dropped or pasted local paths as real uploads when the path points at supported media, while keeping non-media files as @path mentions. It also dedupes the native desktop drop event against the browser drop event so the same file does not get inserted twice.

This is part of the broader Desktop file-drop gap tracked in #18143, where Desktop currently rejects or limits more file types than TUI when inserting dropped files into prompt input.

How did you verify your code works?

• Tried bun typecheck in packages/app, but the workspace currently fails before these changes due to missing local type/runtime deps in this environment (solid-js, @opencode-ai/ui, effect, ghostty-web, and related packages).
• Tried bun test:e2e -- e2e/prompt/prompt-drop-native-event.spec.ts, but this environment is missing @playwright/test.
• Added coverage in packages/app/e2e/prompt/prompt-drop-native-event.spec.ts for native drop image preview and duplicate native/browser drop handling.

Screenshots / recordings

Not included.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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