feat: add month breakdown and session backups

Allow recurring costs and confirmations to remain unassigned when created from a purchase without a post. Add browser backup retention and restore controls, with coverage and documentation updates.

docs/DECISIONS.md

@@ -20,6 +20,10 @@ record.
   ordered migration functions applied on load, because IndexedDB holds the
   user's only copy of their financial data. Now at version 11.
   → [`specs/2026-09-01-budget-app-design.md`](specs/2026-09-01-budget-app-design.md)
+- **Session backups live in browser storage.** Each app start saves a checkpoint
+  before editing is available; 7 daily, 8 weekly and 12 monthly checkpoints
+  are retained. Chosen for automatic operation, with manual JSON export still
+  needed for browser-storage loss. → [`ARCHITECTURE.md`](ARCHITECTURE.md)
 - **A destructive write downloads a backup first.** Import and reset both export
   before replacing, and abort if the export fails. → commit `ac3d18a`
 - **Reset restores first-run state, not an empty dataset.** The seed posts, the
@@ -85,6 +89,10 @@ record.
 
 ## Recurring costs
 
+- **A recurring cost may have no post.** It still appears as due, while no
+  envelope projection moves; confirming it records an unassigned purchase.
+  This lets the purchase dialog turn an unassigned charge into a bill without
+  inventing a destination. → [`ARCHITECTURE.md`](ARCHITECTURE.md)
 - **Two-stage confirmation, over projection-only or generation-only.** A
   recurring cost projects expected occurrences that stay expected until the
   owner confirms one into an ordinary `Purchase`. Projection-only would charge