Add pill buttons for Observation Date on main obs form

[rustynwac]

Replaces the plain date picker for the Observation Date field on the main observation form with pill buttons (Today, Yesterday, Other) and a calendar modal for Other, matching the Occurrence Date behavior on the avalanche observation form.

Fixes #1114

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[rustynwac]

@claude run yarn prettify and commit the changes

[claude]

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• Run yarn prettify
• Commit and push changes

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