Fix header gap around plan picker; disable expand on empty accordions

[bmingles]

Header gap

PlanPicker's trigger could grow wide enough (a long plan name plus the chevron) to overlap the Completed checkbox, since .title spans both grid columns and centers within the full header width — fine for short static titles, not for an arbitrary plan name.

Fix: confine .title to the grid's flexible first column, but only while the picker is actually showing (styles.titleConstrained). This was deliberately made conditional after a first attempt (symmetric padding on .title unconditionally) regressed every route — it truncated even "My Plan", since padding shrinks the box regardless of whether the content actually needs the room. Also added a small column-gap as a guaranteed minimum between the two columns.

Empty accordions shouldn't expand

ChapterGroup's header could toggle open to an empty list: when "Completed" is hidden and every chapter in a group is already read, ChapterGroupList's own group-visibility filter (which only checks search text, not completion) still lists the group, but its filtered chapter count is 0. Guarded the toggle — and added a createEffect to auto-collapse a currently-expanded group if its count drops to 0 — mirroring the identical guard Chapter.tsx already uses for its own read-date accordion (onExpanderClick / dates().length === 0).

Test plan

• npm run build passes
• tsc --noEmit passes
• Measured the header gap directly (bounding boxes): went from -11.6px (actual overlap) to a clean 12px gap
• Verified "My Plan" (previously truncated by the padding attempt) now renders in full, "Books"/"History" titles are pixel-identical to before (still exactly centered, confirmed via bounding-box math), and a long name ("Chronological Reading Journey") truncates gracefully with no overlap
• Reproduced the empty-accordion bug (marked a 1-chapter book fully read, hid Completed, clicked its still-listed header — it expanded to nothing) before the fix, confirmed it no longer expands after, confirmed a book with unread chapters still expands normally (regression check), and confirmed re-enabling "Completed" makes it expandable again

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