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

[raphaelfaouakhiri]

Implements #2126.

Materialized occurrences are currently named by dedup suffix (Pay rent-2, Pay rent-3), which carries no meaning. This PR adds a filename template for them:

• New template variables (occurrence-only): {{occurrenceDate}} (2026-08-01), {{occurrenceWeek}} (2026-W31, ISO), {{occurrenceMonth}} (2026-08), {{occurrenceYear}}, {{occurrenceMonthName}}, and {{occurrencePeriod}} — which auto-selects the granularity from the parent's FREQ (daily→date, weekly→week, monthly→month, yearly→year; complex rules fall back to full date). All regular filename variables also work. Injected via the existing additionalVariables mechanism (same as ICS notes).
• Global setting occurrenceFilenameTemplate (Task properties → Title card). Empty = current behavior (parent title + -N).
• Per-parent override: a frontmatter property on the parent task (name configurable via occurrenceFilenameTemplateProperty, default occurrenceFilenameTemplate) overrides the global template for that task's occurrences. Implemented plugin-side because the core FieldMapping lives in @tasknotes/model.
• The -N dedup suffix is kept as a collision safety net; no template error can abort materialization (falls back to sanitized title); i18n strings added for en + pt.

⚠️ Behavior change — default is ON: the setting ships as {{title}} — {{occurrencePeriod}}, so after updating, newly materialized occurrences get meaningful names by default (existing files are never renamed). I believe this is the better default for users, but if you prefer opt-in, flipping the default to "" in defaults.ts is a one-line change — happy to do it.

Known limitation (pre-existing behavior, unchanged): with storeTitleInFilename enabled, later editing an occurrence's title renames the file to the plain title (the old -N suffix was equally lost).

Tests: new unit suite for the variables/generator + 3 wiring tests in the materialization suite (20 passing); tsc clean; validated in my own vault (daily/weekly/monthly recurrences, global + per-parent override).