#6600 fix(mobile): drop the newline button from the home composer's bottom row

Summary

• #6543 moved the newline button into the terminal's quick-key row but left a fallback copy in the composer's bottom toolbar for surfaces without that row, so the home composer still showed a ↵ button next to +.
• Removes that fallback (newlineButton) and its slot. The terminal's newlineChip in the quick-key row is unchanged.

Test plan

• Biome clean; mobile typecheck has the same 135 pre-existing errors with and without this change, none in GlassComposer.
• Home composer: expand it — bottom row is +, agent picker, mic/send. No ↵.
• Terminal composer: ↵ chip still pinned left of the quick keys and still appends a newline.

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Summary by cubic

Removes the fallback newline button from the home composer’s bottom row in GlassComposer so only the terminal’s quick-key newline chip remains. Previously, the home composer showed a ↵ button beside +; now it does not. The terminal composer is unchanged.

• Home composer: bottom row shows +, agent picker, mic/send; no ↵; no API changes.
• Terminal composer: newlineChip in the quick-key row still inserts a newline.