feat: adjust media gallery viewer

[oliverlaz]

🎯 Goal

Align the media gallery viewer with the REACT-847 mock so the modal chrome, controls, and layout match the updated design.

Linear ticket: https://linear.app/stream/issue/REACT-847/mediagallery-adjustments

🛠 Implementation details

• Added a dedicated GalleryHeader component for the title, timestamp, download action, and close action.
• Updated the gallery styling to use the full available modal area with an edge-to-edge blurred stage, overlay header controls, centered media index, and refined navigation buttons.
• Added the new circular download icon and switched the gallery header to use it.
• Removed the modal overlay close button so the gallery owns its close control.
• Expanded gallery tests to cover the updated header actions and position indicator copy.

🎨 UI Changes

Reference mock: https://linear.app/stream/issue/REACT-847/mediagallery-adjustments

Local comparison screenshot was kept out of the commit and PR branch.

[github-actions]

Size Change: +1.4 kB (+0.32%)

Total Size: 435 kB

Filename	Size	Change
./dist/Channel-BbSF8w4p.js	20.2 kB	+20.2 kB (new file)	🆕
./dist/Channel-CqpzcDQo.js	0 B	-20.2 kB (removed)	🏆
./dist/ChannelPreview-C8OPJzUm.js	14.1 kB	+14.1 kB (new file)	🆕
./dist/ChannelPreview-CBJXfEFg.js	0 B	-14.1 kB (removed)	🏆
./dist/cjs/experimental.js	3.81 kB	-2 B (-0.05%)
./dist/cjs/index.js	217 kB	+563 B (+0.26%)
./dist/css/index.css	44.3 kB	+731 B (+1.68%)
./dist/limits-C0Hh89Qv.js	0 B	-65.2 kB (removed)	🏆
./dist/limits-D8PvuaTc.js	65.3 kB	+65.3 kB (new file)	🆕

ℹ️ View Unchanged

Filename	Size
./dist/audioProcessing-BbOs2wMd.js	1.32 kB
./dist/cjs/emojis.js	2.97 kB
./dist/cjs/mp3-encoder.js	1.27 kB
./dist/css/v2/emoji-mart.css	1.84 kB
./dist/css/v2/emoji-replacement.css	300 B
./dist/css/v2/index.css	39.4 kB
./dist/css/v2/index.layout.css	22.8 kB

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