Move plan picker back out of the title, into its own secondary chip

[bmingles]

Summary

Reverts the "tap the title" plan picker from the previous PR based on feedback: it scaled fine to any number of plans, but taking over the static "Plan" title read as less clear than the original segmented control's distinct, separate-control feel — even though that control itself didn't scale past a couple of plans.

PlanPicker is now a single chip button ("{plan name} ▾") in its own full-width bar above the day list — the segmented control's old position — opening the same checkmarked dropdown menu on tap. The title goes back to always being plain generic text ("Plan"), and Layout's header grid no longer needs the space-reservation logic from the last PR now that there's nothing sharing the title's row. As a side benefit, the chip has a full row to itself with no Completed checkbox competing for width, so it has noticeably more room for long plan names than either earlier design did.

Test plan

• npm run build passes
• tsc --noEmit passes
• Verified: title reads "Plan" regardless of plan count; chip is absent with one plan; with several plans the chip shows the active plan's name, opens a checkmarked menu on tap, updates both the chip label and the day-list content on selection, and is closed by an outside click
• Verified the title stays "Plan" (never repurposed) through all of the above, and that the chip is absent on the Books/History tabs
• Checked a very long plan name in the chip — truncates gracefully with text-overflow: ellipsis, with visibly more room than the previous title-embedded version had

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