Tp-Note v1.26.7

A feature release adding inline Mermaid diagrams and LaTeX math to the
HTML rendition, governed by a new embedded-content error policy, plus
routine dependency upgrades.

- Mermaid code blocks are now rendered to inline SVG in both the viewer
  and the exporter (new `mermaid` build feature, listed in
  `tpnote --version`).
- LaTeX math formulas are rendered to MathML behind a new `latex` build
  feature.
- New embedded-content error policy (`Inline` / `HardError`) controls how
  a failed Mermaid diagram or LaTeX formula is handled: rendered inline as
  an error box (viewer default), or aborting the export in batch mode
  (`tpnote -x . -b`, which always uses `HardError`), so a broken embed can
  never slip silently into an exported HTML or PDF.
- Worked around a label auto-wrapping defect in the Mermaid renderer so
  multi-line node labels are sized and their edges routed correctly.
- Added vertical spacing to rendered content tables for readability.
- Upgraded compatible dependencies.
- Published crates: tpnote-lib 0.46.6, tpnote 1.26.7
  (tpnote-html2md unchanged at 0.3.9).

Installers and standalone binaries for Windows, macOS, Linux and
Debian/Ubuntu are attached below.