Normalize media upload filenames

[manton]

Motivation

• Video uploads were inconsistently named (e.g. video.mov vs original filenames), causing mismatches when uploading and later referencing uploaded media.
• Filenames coming from iOS/Android can contain spaces or unsafe characters that may break upload endpoints or local caching.
• Large/chunked upload flow and single-file upload flow used different filename logic, leading to inconsistencies across the code paths.
• Provide a single, predictable filename strategy to avoid upload/lookup errors and to make cached files safe.

Description

• Added src/utils/file_names.js with inferExtensionFromType, sanitizeFileName, and buildUploadFileName helpers to infer extensions and clean filenames.
• Updated large upload flow to use buildUploadFileName and to use sanitized target names in ensure_local_uri_for_upload so cached copies use safe filenames.
• Updated MicroPubApi.upload_media to use buildUploadFileName when appending the file to FormData so single-file uploads use the same cleaned filename.
• Removed an unused Platform import from MicroPubApi.js as part of the change.

Testing

• No automated tests were run against these changes.
• (No unit or integration tests were executed as part of this PR.)
• Manual verification is recommended for both single-file uploads and large/chunked uploads across iOS and Android to confirm filename behavior.
• Linting/build were not executed here.

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Codex Task