feat: configurable filename template for materialized occurrences (#2126)

[raphaelfaouakhiri]

Implements #2126.

Adds optional filename templates for materialized recurring-task occurrences:

• Explicit occurrence variables: {{occurrenceDate}}, {{occurrenceWeek}}, {{occurrenceMonth}}, {{occurrenceYear}}, and {{occurrenceMonthName}}. All regular filename variables also work.
• A global setting under Task Properties → Title. It is empty by default, preserving the existing parent-title-plus-numeric-suffix behavior until a user opts in.
• A configurable per-parent frontmatter override, defaulting to occurrenceFilenameTemplate.
• Existing collision suffixing and safe fallback behavior. Existing files are never renamed.

TaskNotes deliberately does not infer filename granularity from the recurrence rule; users choose the appropriate occurrence variable explicitly.

Includes localized settings text, user documentation, an unreleased entry, and regression coverage for variables, global and parent templates, legacy behavior, and the opt-in default.