Epic: Block composition & organization

[ApocDev]

Make blocks easier to structure and arrange: reorder rows to match how a chain is actually thought about, collapse a long chain into a nested sub-block, organize blocks into nested folders, and toggle parts of a plan on and off.

Sub-issues:

• Tasks & notes — checklists linkable to blocks, assistant-aware #5 — tasks & notes: checklists linkable to blocks, assistant-aware (also feeds the planning model, Epic: Richer planning model #31).
• Block view: drag/drop to reorder recipe rows #6 — drag and drop to reorder recipe rows within a block.
• Sub-blocks: collapse a multi-step chain into a nested block #7 — sub-blocks: collapse a multi-step chain into a nested block (shipped display-only).
• Sidebar: nested folders (folders inside folders) #8 — nested sidebar folders (folders inside folders).
• Sidebar: show a drop indicator while dragging blocks/folders #37 — show a drop indicator while dragging blocks/folders in the sidebar.
• Custom block icons; drop the implicit "primary goal" #40 — custom block icons; drop the implicit "primary goal".
• Enable/disable individual recipes and whole blocks (as toggles) #73 — enable/disable individual recipes and whole blocks (as toggles).
• Sub-blocks v2: real composition — separately-solved modules with hidden internal goals #76 — sub-blocks v2: real composition (separately-solved modules, hidden internal goals).
• Plan snapshots: block-level history, restore, and diff #85 — plan snapshots: block-level history, restore, and diff.