Fix unhandled DateTimeParseException on malformed date query params

[Copilot]

Malformed startDate/endDate query params (e.g. /compare?startDate=not-a-date) caused an unhandled DateTimeParseException, surfacing as a 500.

Changes

• Parameter types: Replaced String + manual LocalDate.parse() with @DateTimeFormat(iso = DateTimeFormat.ISO.DATE) LocalDate — Spring MVC owns parsing, invalid input is caught at binding time instead of mid-method.
• Error handling: Added @ExceptionHandler(MethodArgumentTypeMismatchException.class) that returns a user-facing message on the index page naming the offending param, with form defaults repopulated.

// Before — throws unhandled DateTimeParseException on bad input
@RequestParam(required = false) String startDate,
LocalDate start = (startDate != null && !startDate.isBlank()) ? LocalDate.parse(startDate) : ...;

// After — Spring handles parsing; type mismatch caught by @ExceptionHandler
@RequestParam(required = false) @DateTimeFormat(iso = DateTimeFormat.ISO.DATE) LocalDate startDate,
LocalDate start = (startDate != null) ? startDate : LocalDate.now().minusYears(1);

The exception handler distinguishes date params from other type mismatches, returning e.g. "Invalid date format for 'startDate'. Please use YYYY-MM-DD (e.g. 2024-01-15)." rather than a generic or HTTP error response.

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[lukepring]

@copilot check the merge conflict ty x

[Copilot]

@copilot check the merge conflict ty x

Checked against base Sprint-2-Jack; branch merges cleanly with no conflicts (no new commit required).