Replaces the attachments UI from moving the text input box aside to popping up a menu instead:

before

after

Also fixes text field vertical centering issue under Cupertino:

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after

issues fixed by this PR

• Input box shrinks unexpectedly when clicking file attachment button – customization not supported by Flutter AI Toolkit. #90

• Cupertino render doesn't center text input inside text input area #92

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read the [Flutter Style Guide] recently, and have followed its advice.
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.