fix: preserve negative sign when editing an expense

[krokosik]

Description

This PR fixes two issues with the edit-expense flow.

1. Edit form shows empty/add flow instead of the expense (regression)

The initializedExpenseIdRef / initializedGroupIdRef / initializedFriendIdRef refs were introduced in 83268ac (v2.1.0, #608) to prevent re-initialization on PWA reconnect. They persist across React StrictMode's simulated unmount/remount in dev mode, while the existing resetState() cleanup at add.tsx:47 clears the store. On StrictMode remount, the edit effect sees ref.current === expenseId and early-returns, leaving the store empty and the form rendering the add flow ("select members", all fields empty) even though the URL has the correct expenseId and the cached expenseQuery.data is populated. A hard reload works around it because it clears the React Query cache so the effect doesn't run until data arrives, by which time StrictMode is done.

Fix: clear the three refs alongside resetState() in the unmount cleanup so refs and store are always reset together. Safe in production (refs are destroyed on real unmount anyway).

2. Negative expenses shown as positive when editing (#658)

The edit form loaded the amount string with toUIString(amount, false, true) (signed=false), which strips the minus, while setAmount(amount) correctly set isNegative=true. The store was left inconsistent: the input showed 200 but the model said "negative 200". Any digit edit re-derived isNegative from the now-positive typed BigInt, flipping the sign on save. If the user didn't touch the field, isNegative stayed true and the value was unchanged — matching the reporter's repro exactly.

The sign was lost at multiple layers, so the fix threads signed through all of them:

• src/utils/numbers.ts — normalizeToMaxLength now accepts and forwards signed to sanitizeInput; the string branch of parseToCleanString forwards signed. The bigint branch keeps its external sign prefix (no double-minus). Removed the unused toUIStringSigned helper.
• src/pages/add.tsx — load the edit form with toUIString(amount, true, true) so amountStr = '-200' matches isNegative = true.
• src/components/ui/currency-input.tsx — pass allowNegative to parseToCleanString on focus so the minus is not re-stripped when the field gains focus.
• src/components/AddExpense/AddExpensePage.tsx — keep the sign when converting currencies on a negative expense (onConvertAmount).
• src/components/Friend/CurrencyConversion.tsx — same fix in two places.
• src/components/Friend/Export.tsx — parseToCleanString(expense.amount, true) so CSV export of negative expenses keeps the minus (same root cause, surfaced during investigation).

After the fix, editing a -200 expense shows -200 in the field. Changing digits to -200.01 preserves the sign; deleting the minus and saving flips it to +200.01 (explicit user intent).

Tests

• src/tests/number.test.ts — added toUIString signed + hideSymbol cases, and a new parseToCleanString describe block covering signed string and bigint inputs (plus the default signed=false behavior).
• src/tests/addStore.test.ts — regression tests driving the real Zustand store: loading a negative expense keeps the minus in amountStr + isNegative=true; editing digits without removing the minus preserves the sign; deleting the minus flips it.

Verification

• pnpm prettier --check . — clean (only a pre-existing .opencode/package.json warning, unrelated)
• pnpm lint — 0 errors, 284 pre-existing warnings
• pnpm tsgo --noEmit — clean
• pnpm test — 155/155 pass
• pnpm build --no-lint — success
• Manual: editing an expense now populates the form without requiring a refresh; negative amounts keep their sign across edit cycles

Closes #658

Checklist

• I have read CONTRIBUTING.md in its entirety
• I have performed a self-review of my own code
• I have added unit tests to cover my changes
• The last commit successfully passed pre-commit checks
• Any AI code was thoroughly reviewed by me