fix: pre-fill full remix parent when entering contest from mobile

[dylanjeffers]

Summary

• Mobile's "Enter Contest" button (ContestScreen.handleEnterContest) was only passing parent_track_id into the upload flow, dropping the user object, the has_remix_author_reposted / has_remix_author_saved flags, the source-track artwork, and the genre — fields that the existing "Upload Your Remix" button already pre-fills. The upload form's remixed-from context rendered incomplete on contest entries.
• Extracted the metadata shape into a shared useEnterContest hook in @audius/common/hooks. Web and mobile each have a thin wrapper that handles platform-specific artwork blob fetching (Web File vs RN's _data shape) and navigation, but the remix_of shape is now defined in one place.
• Deduped the inline copy in the desktop RemixContestSection so all four entry points (web track page, web contest page, mobile track screen, mobile contest screen) share one source of truth.

Affected entry points

Platform	Component	Before	After
Web desktop track page	RemixContestSection	inline impl	uses shared hook
Web mobile/contest page	useEnterContest	already correct	now wraps shared hook
Mobile track screen	UploadRemixFooter	inline impl	uses shared hook
Mobile contest screen	ContestScreen.handleEnterContest	only parent_track_id	uses shared hook

Test plan

• Mobile: open a track that hosts a remix contest → tap Enter Contest → confirm upload form shows the contest track's title, artist name, artwork thumbnail, and genre pre-filled in the remix section
• Mobile: same track → tap Upload Your Remix from the track screen footer → confirm identical pre-fill (regression check on the established flow)
• Web desktop: visit a contest page → click Enter Contest → confirm upload form pre-fills artwork/genre/remix-of (regression check; uses same hook)
• Web desktop: visit a track page hosting a contest with feature flag off → click Upload Your Remix in RemixContestSection → confirm pre-fill (regression check on deduped inline impl)
• Confirm the "remixed from" tile in the upload form shows author avatar + name (this is what the missing user field broke on mobile)

🤖 Generated with Claude Code

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