feat: timeline continues across midnight (prev/next day) (v0.7.1)

The vertical timeline used to flat-cut at 0/24h, hiding the high/low just
before/after midnight (e.g. 노력항 6/6 showed 간조-만조-간조 with the next
high lost). Now it carries the curve through both ends:
- proxy: TideDay gains prevEvent (last extremum before midnight) and
  nextEvent (first after). Model derives them from the already-stitched ±1
  day zeta; official fetches the neighbor days' edge events. fmtTime handles
  date rollover.
- app TideTimeline: the curve is drawn over [prev, ...today, next] mapped to
  hours −24..+48 so it enters/exits smoothly instead of cutting flat; dimmed
  "어제 HH:MM 만조/간조" and "내일 …" pills mark the boundary extrema.

Verified: 군산/노력항 return prev/next; timeline curve no longer flat at the
edges and shows the carry-over labels.

Constraint: edge events are display-only continuation; today's events list/range are unchanged.
Confidence: high
Scope-risk: module
Reversibility: trivial
Tested: vitest 28/28; app build; Playwright shows continuous curve + 어제/내일 boundary labels.
Not-tested: on-device; visual crowding when a today-event sits within minutes of midnight (군산 00:26 case).
Related: docs/BACKLOG.md 5

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