feat(wave-admin): UI for managing waves; split current/upcoming display

[efstajas]

Summary

Front-end for the new wave-admin endpoints (see matching wave PR: drips-network/wave#624), plus a small fix to the public wave program page so a current wave and an upcoming wave can both show at the same time.

Admin

• New "Waves" card on /wave/admin (gated on manageWaves).
• /wave/admin/waves — lists wave programs with a "Manage Waves" link each.
• /wave/admin/waves/[waveProgramId] — lists scheduled waves with status badges and per-row actions:
  • Schedule wave (header action) opens a modal with start/end dates and a budget input pre-filled from the program's preset.
  • Edit (upcoming) opens the edit modal with all fields editable.
  • Extend (active) reuses the edit modal but locks start date + budget; end-date picker is clamped via `min` to the current end and validates inline.
  • Delete (upcoming only) — destructive variant, behind a confirmation modal + standard error-modal wrapper.
  • Ended waves are read-only — no actions.
• New API client helpers in `wavePrograms.ts`: `manualCreateWave`, `updateWave`, `deleteWave`.

Public wave program page

• Previously: `upcomingWave ?? activeWave` collapsed both into one slot, so a program with both an active wave and an upcoming wave showed only the upcoming one.
• Now: separate "Current Wave" and "Upcoming Waves" sections.
• Upcoming section is hidden when there's a current wave but nothing scheduled (matches the user's preference — no noisy "nothing here" card alongside the live wave).
• The existing newsletter/Discord empty state is preserved for the no-active + no-upcoming case.

Test plan

• As a user with the `manageWaves` permission, open `/wave/admin` and confirm the new "Waves" card appears.
• As a user without it, confirm the card does not appear and `/wave/admin/waves` redirects to `/wave/admin`.
• Schedule a new wave — confirm 201 and that it appears in the list with status "upcoming".
• Edit an upcoming wave — change start date, end date, budget independently.
• Try to edit an active wave — confirm start + budget are disabled, end-date picker cannot select a date earlier than the current end.
• Try to edit an ended wave — confirm no edit button shows.
• Delete an upcoming wave — confirmation, success toast, list refresh.
• Confirm no delete button on active / ended waves.
• Public page: navigate to a wave program with an active wave but no upcoming → only "Current Wave" shows.
• Public page: with active and upcoming → both sections show, current on top.
• Public page: with no active or upcoming → the newsletter/Discord empty state still renders.