Adds a `floatingAction` slot to PageShell. Desktop renders it inline
next to the H1 (same as before); mobile drops the entire header row
and floats the action as a fixed pill in the bottom-right corner —
the page now starts straight at content with no wasted vertical
space at the top when only an action exists.
Add Account / New Reminder buttons grow to size-12 circles on mobile
(easy thumb target) and keep the compact h-7 inline pill on desktop.
The action node is rendered twice in the tree — once inline, once
fixed — and switched via responsive utilities.
Bumps mobile bottom padding to pb-20 when a FAB is present so the
last card doesn't sit under the floating button.
Activity's "Clear history" still uses the regular `action` slot — it
keeps the inline header row on mobile because it isn't the page's
primary CTA.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
yiekheng
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