feat: template substitution for note bodies, filenames, and folders

[Chusca]

Summary

Adds a small template engine so a Type can control how its notes are written, named, and filed at creation time. Three frontmatter fields share one resolver:

• template - note body, expanded with {{date}}, {{time}}, {{type}}
• _filename_template - token-based filename (overrides the default slug)
• _subfolder_path - token-based, vault-relative folder for new notes

Tokens accept an optional date-fns format specifier, e.g. {{date:yyyy-MM-dd}}, {{date:EEEE}}, {{time:HH:mm}}. Unknown or malformed tokens are left verbatim so typos stay visible and predictable.

Why

Date-aware notes (daily journals, meeting logs) should work out of the box - the same affordance I'm used to from apps like Logseq. Token-based filing (journals/{{date:yyyy}}/{{date:MM}}) keeps a time-series organized without per-note effort.

What's in this PR

Template engine (src/utils/templateSubstitution.ts) - pure substituteTemplate(template, ctx) backed by a TOKEN_RESOLVERS map for date / time / type. Shared by body, filename, and folder substitution.

Body templates - template: is expanded in buildNoteContent during note creation.

_filename_template - feeds resolveTypeFilename, overriding the slug in resolveNewNote. If the resolved path already exists, creation returns an existing plan and opens the note instead of blocking.

_subfolder_path (src/utils/typeSubfolder.ts, new) - Type-scoped folder for new notes:

• Pure helpers: resolveTypeSubfolder, sanitizeSubfolderPath, effectiveImmediateFolder
• Sanitization strips illegal cross-platform chars (< > : " | ? * \ + control chars), drops .. / . segments so the path cannot escape the vault, trims edges, and returns null when nothing usable remains
• Folder precedence: explicit user-chosen folder > type subfolder > vault root (an explicit empty string is treated as an intentional root override and preserved)
• Resolved at creation time only - existing notes are never moved; same-day dedup becomes folder-scoped
• Mirrors the _filename_template plumbing end-to-end: subfolderPath is parsed in Rust (VaultEntry.subfolderPath over IPC) and registered in the known-frontmatter-key table

_filename_template and _subfolder_path are underscore-prefixed system properties (per ADR-0008), configurable via raw frontmatter only - no UI field in the Type customize modal.

Example

A daily-journal Type using all three fields:

---
type: Type
title: Journal
_filename_template: "{{date}}"
_subfolder_path: "journals/{{date:yyyy}}/{{date:MM}}"
template: |
  # {{date:EEEE, MMMM do, yyyy}}
  ## What happened today?
---

Creating a new journal on 2026-06-07 files it at journals/2026/06/2026-06-07.md with body:

# Sunday, June 7th, 2026
## What happened today?

Analytics

• note_created is augmented with used_filename_template and used_subfolder_path
• note_opened_existing fires when a filename-template collision opens an existing note

Test plan

• pnpm exec vitest run src/utils/templateSubstitution.test.ts - tokens, format specifiers, edge cases
• pnpm exec vitest run src/utils/typeSubfolder.test.ts - 16 tests (resolution, sanitization, precedence)
• pnpm exec vitest run src/hooks/useNoteCreation.test.ts src/hooks/useNoteCreation.extra.test.ts - both creation paths, filename resolution, folder routing, collision/open-existing
• cargo test --manifest-path src-tauri/Cargo.toml - subfolderPath parse/serialize + frontmatter-key registration
• Playwright journal smoke (extended) covers end-to-end filing
• pnpm lint · npx tsc --noEmit · pnpm test - clean
• Coverage gates: frontend ≥70%, Rust ≥85%

Notes for reviewers

Two commits fix problems surfaced while dogfooding the feature end-to-end (both ship with regression tests):

• Cache version bump (52a40b6) - CACHE_VERSION goes 14 → 15 so a stale on-disk cache from before these VaultEntry fields shipped is invalidated once; every Type's templates are then parsed on the first launch after upgrade. Without it, the first note of each type would render with an empty body and land at the vault root until a later rescan repopulated the fields.
• Block-scalar templates (4b04aa) - the JS frontmatter parser now correctly captures YAML literal/folded block scalars (template: | / >), the natural way to author a multi-line template, and _subfolder_path is mapped through the same frontmatter-key table as _filename_template.