Stage 3: Plan editor with Steps, Supersets and reordering

- Plan model with immutable edits: add with Mode defaults, move, remove,
  Superset with next, take a member out, ungroup; Stretching length
  estimate from holds, sides, rounds and Transitions
- drift database (plans table, items as JSON, ADR 0005) and
  PlanRepository; Plans list per Mode with rename/duplicate/delete
- Editor: drag handles, step editor sheet (sets, reps or time, optional
  kg; hold, both sides, rounds; Superset sets and rest between),
  pause between holds for Stretching Plans
- Exercise picker reusing the Library browser; Add to plan from the
  Exercise detail page
- Theme: Mode-styled snackbar, FAB, menus, segmented buttons, switches

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/adr/0005-plan-items-stored-as-json.md

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+# ADR 0005 — A Plan's Steps and Supersets are stored as JSON
+
+Status: Accepted (2026-10-29)
+
+A Plan's items (Workout Steps, Stretching Steps and Supersets with their members) are stored as one
+JSON column on the `plans` row, following CookIT's ADR 0002. A Plan is always loaded, edited and saved
+as a whole, every edit is an immutable `Plan.withX` operation that can be unit-tested without a
+database, and reordering or grouping into Supersets needs no position or group bookkeeping.
+
+Plans are only listed per Mode, newest first.
+
+## Consequences
+
+- Items carry no ids; the editor keys rows by object identity, which is enough because every save
+  re-reads the Plan.
+- Sessions will store a snapshot of the Plan's items (same JSON), so editing a Plan never changes
+  History.
+- Querying across Plans by Exercise in SQL would need a table of its own; nothing needs it yet.