docs: capture durable product truth in PRODUCT.md

Written via the impeccable init flow. Records what future work must not
reinvent: a permanently single-user tool for one person managing their own
money, offline-only with no account, and five product principles including two
the owner added directly — changing an allocation must never destroy history,
and splitting a purchase across posts must stay easy.

Evidence on Hand deliberately records absences: no customers, testimonials,
usage data or integrations exist, and the only real content is the owner's own
budget data, which never leaves their browser. Future surfaces must not
manufacture them.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

PRODUCT.md

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+# Product
+
+<!-- impeccable:product-schema 1 -->
+
+## Platform
+
+web
+
+## Users
+
+One person: the owner of the money, managing their own budget. They are leaving
+a spreadsheet, so they already know their own categories, already think in
+months, and value speed of entry over guidance. They tolerate rough edges in
+exchange for the tool doing exactly what they mean.
+
+Confirmed permanently single-user. One person, one browser, one dataset. There
+are no accounts, no second role, and no shared or household use planned.
+
+## Product Purpose
+
+Envelope budgeting for a single person, replacing a spreadsheet.
+
+Money is divided each month into named envelopes ("posts"). Unspent allocation
+carries into the next month; overspending carries forward as debt. The app
+answers two questions at a glance: how much is left in this envelope, and did I
+overspend.
+
+Success is that the owner stops maintaining a spreadsheet, and that a month's
+purchases can be entered fast enough not to be a chore.
+
+## Positioning
+
+Two things a general budgeting app does not do:
+
+- **Envelope rollover is the whole model, not a feature.** Every figure derives
+  from one left-fold across months, so a balance is always the honest
+  consequence of every prior month.
+- **The data never leaves the browser and never needs an account.** IndexedDB is
+  the only store. That is a deliberate product position, not a missing backend.
+
+## Operating Context
+
+The owner records spending after the fact, often in batches — a grocery trip is
+twenty lines entered in one sitting, not twenty separate visits. Income arrives
+monthly and varies, so it is entered per month rather than assumed.
+
+Purchases frequently belong to more than one envelope (food bought at a social
+event), and larger purchases are spread across several months deliberately
+rather than absorbed by one.
+
+Money is handled in DKK, USD and EUR, with DKK as the base.
+
+## Capabilities and Constraints
+
+Confirmed and in place:
+
+- Posts (envelopes) with monthly allocation, created and archived by the owner.
+  Posts are archived, never deleted, because purchases reference them.
+- Allocation is either a fixed amount or a percentage of that month's income.
+- A purchase can be split across several posts, by percentage or by fixed
+  amounts, and can also be spread across future months as a payment plan. The
+  two compose.
+- Per-month income, entered directly, varying month to month.
+- Multi-currency (DKK/USD/EUR) against a user-maintained exchange-rate table,
+  with an optional on-demand fetch that fails soft and is never required.
+- JSON export/import and ODS (spreadsheet) export, both generated in the
+  browser.
+- Offline-capable installable PWA. Currently deploys as one static artifact to
+  both GitHub Pages (under a subpath) and a self-hosted Docker image.
+
+Terminology: an envelope is called a **post**. A **finance plan** is a purchase
+spread across months. An **override** is a one-month exception to a post's
+allocation.
+
+Undecided: nothing outstanding at the product level.
+
+## Brand Commitments
+
+The product is called **Budget 2.0**. No logo, wordmark, or brand assets exist,
+and none have been commissioned.
+
+## Evidence on Hand
+
+None, and none may be invented. This is a personal tool: there are no
+customers, testimonials, case studies, usage statistics, press, pricing, or
+third-party integrations. Any future surface must not manufacture them.
+
+The only real content is the owner's own budget data, which never leaves their
+browser and is therefore never available as example material. Illustrative data
+must be visibly synthetic.
+
+## Product Principles
+
+1. **Over-budget is always allowed.** The app reports the truth and never
+   blocks, clamps, or hides overspending. A negative balance is a normal value.
+2. **Changing an allocation must never destroy history.** Adjusting what a post
+   gets going forward leaves what it got before intact and still visible.
+3. **Splitting a purchase across posts must stay easy.** It is a routine act,
+   not an advanced feature, and must not be buried behind extra steps.
+4. **The data stays portable and in the owner's hands.** Export is always
+   available and always complete; nothing traps a budget inside the app.
+5. **Entry speed is a feature.** Recording a month of spending is the most
+   repeated act in the product, and friction there is the reason spreadsheets
+   get abandoned.
+
+## Accessibility & Inclusion
+
+No condition-specific requirement was established by the owner. The existing
+implementation holds itself to WCAG AA contrast, visible keyboard focus, and
+respecting reduced-motion preferences; keyboard-first entry is a stated goal of
+the fast-entry flow rather than only an accessibility accommodation.