Couple daily can and spend limits via usual can size

Link daily caps through the user's chosen standard can size (250/355/473ml)
and built-in prices. Onboarding asks for size and can ceiling with derived
spend preview. Settings syncs cans and spend bidirectionally. Forecast lock
button sets both limits together.

Co-authored-by: nh9961 <hello@nedhalksworth.com>

src/lib/metrics.ts

@@ -1,4 +1,4 @@
-import type { RedBullEntry } from "../types";
+import type { BuiltInSize, RedBullEntry } from "../types";
 
 export const CAFFEINE_PER_250ML = 80;
 export const SUGAR_PER_250ML = 27;
@@ -8,6 +8,21 @@ export const STANDARD_CAN_VALUES = {
   473: { pricePerCan: 2.85, caffeineMg: 151 },
 } as const;
 
+export const BUILT_IN_SIZES: BuiltInSize[] = [250, 355, 473];
+
+export function priceForLimitSize(size: BuiltInSize): number {
+  return STANDARD_CAN_VALUES[size].pricePerCan;
+}
+
+export function spendLimitFromCans(cans: number, size: BuiltInSize): number {
+  return Math.round(cans * priceForLimitSize(size) * 100) / 100;
+}
+
+export function canLimitFromSpend(spend: number, size: BuiltInSize): number {
+  const raw = spend / priceForLimitSize(size);
+  return Math.round(raw * 4) / 4;
+}
+
 export function spendFor(entry: RedBullEntry) {
   return entry.cans * entry.pricePerCan;
 }

src/lib/userLimits.ts

@@ -7,6 +7,9 @@ export const DEFAULT_LIMITS: UserLimits = {};
 const PREFS_CAN_KEY = "dailyCanLimit";
 const PREFS_SPEND_KEY = "dailySpendLimit";
 const PREFS_STOP_KEY = "stopTime";
+const PREFS_SIZE_KEY = "limitCanSizeMl";
+
+const VALID_LIMIT_SIZES = new Set([250, 355, 473]);
 
 export function parseUserLimits(prefs: Record<string, unknown> | null | undefined): UserLimits {
   if (!prefs) return { ...DEFAULT_LIMITS };
@@ -16,9 +19,12 @@ export function parseUserLimits(prefs: Record<string, unknown> | null | undefine
   const spendLimit = Number(prefs[PREFS_SPEND_KEY]);
   const stopTime = typeof prefs[PREFS_STOP_KEY] === "string" ? prefs[PREFS_STOP_KEY] : undefined;
 
+  const sizeLimit = Number(prefs[PREFS_SIZE_KEY]);
+
   if (Number.isFinite(canLimit) && canLimit > 0) limits.dailyCanLimit = canLimit;
   if (Number.isFinite(spendLimit) && spendLimit >= 0) limits.dailySpendLimit = spendLimit;
   if (stopTime && /^\d{2}:\d{2}$/.test(stopTime)) limits.stopTime = stopTime;
+  if (VALID_LIMIT_SIZES.has(sizeLimit)) limits.limitCanSizeMl = sizeLimit as 250 | 355 | 473;
 
   return limits;
 }
@@ -34,6 +40,9 @@ export function serializeUserLimits(limits: UserLimits): Record<string, unknown>
   if (limits.stopTime) {
     data[PREFS_STOP_KEY] = limits.stopTime;
   }
+  if (limits.limitCanSizeMl != null && VALID_LIMIT_SIZES.has(limits.limitCanSizeMl)) {
+    data[PREFS_SIZE_KEY] = limits.limitCanSizeMl;
+  }
   return data;
 }
 
@@ -45,6 +54,7 @@ export function mergePrefsWithLimits(
   delete next[PREFS_CAN_KEY];
   delete next[PREFS_SPEND_KEY];
   delete next[PREFS_STOP_KEY];
+  delete next[PREFS_SIZE_KEY];
   return { ...next, ...serializeUserLimits(limits) };
 }