feat: refine month totals and recurring purchase entry

docs/specs/2026-10-03-month-and-purchase-refinements.md

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+# Month and purchase refinements
+
+The owner requested this round on 2026-10-03, with the work queued in TODO
+before implementation. The follow-up added a visible Total-field reset bug.
+
+## Actual available
+
+The month header shows entered income minus actual spending beside Spent.
+This includes unassigned purchases, refunds, foreign-currency charges and the
+selected month's finance-plan slices. Unpaid recurring occurrences do not
+reduce it. Allocation and envelope carry do not enter this calculation.
+Negative values remain visible, and arithmetic and display use dataset digits.
+
+Purchases before the envelope fold's start still count in Spent and Actual
+available. The month view uses the existing charge calculation for those
+months; the rollover fold continues to begin at its configured start.
+
+## Display choices
+
+Monthly breakdown has a Show chart checkbox. Its header and checkbox remain
+visible when the chart and detail list are hidden. Expected has an Expand /
+Collapse button and shows counts for expected, coming-up and overdue entries
+when closed. Closing it preserves any typed confirmation amount.
+
+Both choices default to visible/expanded and persist across months and reloads
+in browser localStorage. This follows the theme decision: display preferences
+belong to this browser, not to the exported budget. Blocked localStorage must
+not stop the app or the controls from working. Neither choice changes money,
+occurrences or confirmation behavior.
+
+## Purchase entry
+
+New purchases begin with Recurring cost set to None. Their recurring option
+creates a new bill from this purchase, rather than selecting an existing bill.
+Creating a bill and its first charge remains one atomic mutation. Calendar
+slots and last-charge date requirements retain their existing rules.
+
+Settings → Recurring costs includes a checkbox to allow selecting existing
+bills in Add purchase. The optional dataset boolean is
+`settings.allowExistingRecurringCost`; absent means false, so existing budgets
+adopt the new default without a migration. JSON import accepts only booleans
+when this field is present. The choice travels with a JSON export.
+
+Editing a linked purchase continues to show its saved association even with
+the setting off, and choosing None can unlink it. Ordinary purchases do not
+automatically become recurring bills. This resolves the default-selection
+ambiguity in favor of an explicit recurring action.
+
+The Total field has its own text state to preserve partly typed money inputs.
+Resetting the dialog now resets that text alongside the numeric draft: new
+purchases start blank and existing purchases start with their saved amount.
+The expanded new-bill form exceeded a phone viewport in a browser check. The
+purchase dialog now caps its height to the viewport and scrolls its contents,
+keeping both the first fields and Save reachable.
+
+## Default currencies
+
+New datasets include Japanese yen (JPY, ¥), Australian dollars (AUD, AU$),
+Canadian dollars (CAD, CA$) and Mexican pesos (MXN, MX$). Existing tables are
+not migrated: the request specified defaults, and editing an established
+table is the owner's action. Every currency still uses dataset-wide precision.
+
+The build's FX targets derive from the seed table. All four have committed
+fallback rates as well as live build-time fetching. The committed rates were
+obtained from Frankfurter's DKK-base quote dated 2026-10-02 and inverted to
+DKK per unit, preserving six decimal places. Like the previous constants,
+these rows are marked manual and seed only new datasets.
+
+Source: https://api.frankfurter.dev/v1/2026-10-02?base=DKK&symbols=JPY,AUD,CAD,MXN
+
+The amount parser strips longer symbols before the plain dollar sign, so
+AU$, CA$ and MX$ work without leaving stray letters in the amount. An explicit
+currency code still disambiguates a symbol collision.
+
+## Build stamp
+
+Colophon shows the source ref and a separate build-time UTC date. An exact
+commit keeps its source link; an uncommitted checkout retains the dirty suffix.
+Source archives say source archive rather than inventing a Git ref, while
+the service worker still receives a unique cache version. Development says
+development and no build date. Browser accessors tolerate an absent process
+global, following the existing Bun environment-inlining rule.
+
+## Verification
+
+Regression coverage exercises the actual money calculation, purchase save /
+reopen, new recurring creation, enabled existing-bill selection, linked editing,
+display persistence, collapse/expand preserving a typed amount, Settings write
+errors, JSON preference validation, symbol parsing and new seed conversion.
+Chrome checks verify the month controls, saving and reopening a purchase,
+persisting the Settings choice to IndexedDB, creating a new bill and its first
+charge together, keeping mobile Save reachable, and production build metadata.
+Desktop and mobile captures cover light and dark themes. Screenshots establish
+layout and console behavior; visual feel still requires the owner.
+The complete suite passes 911 tests across 66 files, TypeScript is clean, and
+production builds succeed at both root and subpath base URLs with seed rates.