[FEAT](Sim Planner) Purchases: submodule totals global budget UX

[BenBotros]

🎯 Feature Overview (Why?)

PRD #1555: "Either the submodule totals OR the Global budget must be
filled." The backend enforces the XOR and the one-entry-per-category rule at
creation (422 codes, planner_purchase = 81 / planner_purchase_budget =
82), but the UI shows both submodules side by side with no exclusivity
affordance — the user discovers the rule only via a raw error.

Design ↔ implementation divergence. The PRD/design screenshot shows the
submodule totals table and the Global budget field side by side, as if
both can be filled together — but the backend is XOR, so that layout
actively misleads. Mechanically disabling one half whenever the other holds
data makes the screen read as broken, not as a designed either/or. Properly
expressing the XOR needs a reworked design / new UI frame for the
Purchases section, decided with product/design — not bolted onto the
existing side-by-side layout. That design rework is the gating decision for
this feature; the items below are the interaction requirements it must
satisfy plus a candidate approach, not a settled UI.

🧩 Solution

Blocked on a design rework (see Overview): the side-by-side layout and
the XOR backend diverge, so the section needs a new design frame before
this UX is built. Park the design decision for product/design per the
guardrails. The items below are the
requirements that frame must satisfy plus a candidate mechanical approach,
not a settled UI.

• In the planner Purchases module (bespoke plannerPurchase config,
  planner-module-config/module-configs.ts): the two modes (per-category
  submodule totals vs global budget) must be a visible, explicit
  either/or — the reworked design decides the affordance (e.g. a segmented
  control / mode toggle) rather than leaving the user to infer the rule from
  greyed-out fields.
• Candidate mechanical fallback if the design keeps the current layout: when
  a global-budget entry exists, disable the per-submodule table/form with an
  explanatory hint — and vice-versa. Deleting the blocking entry re-enables
  the other mode without reload (both submodules' data live in the same
  module fetch).
• Map the backend 422 error codes to i18n messages (en+fr) surfaced inline
  on the form, not as a raw-JSON toast — covers the race where two tabs
  create a budget and a submodule row concurrently (the second gets the
  server 422 despite UI gating).
• One-entry-per-category duplicate rule surfaced inline on the category
  select (disable already-used categories or map the 422).
• Missing-factor display: entries carry no kg CO₂eq until the average-EF
  factor rows are uploaded (data concern, not code) — show an em-dash or
  explanatory tooltip instead of a misleading hard 0.
• Guard: reference-kg column and % slider must never appear for Purchases
  (manual type — no snapshot rows).

📋 Implementation Plan

• Design rework decided with product/design first — a Purchases UI
  that expresses the XOR as an explicit either/or (mode affordance);
  gates the items below
• Mutual-exclusion state derived from the module's entries; disable the
  opposite mode's form/table with hint
• Re-enable on deletion of the blocking entry (no reload)
• Enumerate the backend 422 codes from modules_planner/purchase and
  map each to an i18n message (en+fr), rendered inline
• Duplicate-category rule surfaced on the category select
• Missing-factor kg display convention (— / tooltip) per clarification
• Amount edited to 0 still counts as "filled" (blocks the other mode) —
  deleting is the way to switch modes; hint says so
• Verify no reference columns/slider render for Purchases

⚙️ Clarifications Needed

• Design rework (hard blocker): the side-by-side design and the XOR
  backend diverge — a new/reworked Purchases design that expresses the
  either/or (mode affordance) is needed before the UX is built. Owned by
  product/design; gates the items below.
• Exact 422 error-code list (enumerate from
  backend/app/modules_planner/purchase/ in review).
• Missing-factor kg: — vs 0 + tooltip?
• Switching modes: require explicit deletion (recommended, matches backend),
  or offer a "switch to global budget" action that deletes for you (needs
  confirm)?