chore: bump Zola to 0.22.0 (Giallo highlighting migration)

[n-WN]

Summary

Bumps the build toolchain from Zola 0.21.0 to Zola 0.22.0 and performs the required highlighting-config migration.

Zola 0.22.0 replaces the Syntect syntax highlighter with Giallo (see the getzola/zola CHANGELOG). This is a breaking change: all highlighting options move from [markdown] into a new [markdown.highlighting] table, and highlight_theme is gone. Without this migration the site fails to build (unknown field highlight_theme / unknown field highlight_code).

Production deploys ship the artifact built by deploy-cloudflare-pages.yml (Cloudflare Pages does not rebuild), so that workflow's Zola version is authoritative and is bumped here. test-build.yml is aligned to the same version.

Config change (config.toml)

Before:

[markdown]
highlight_code = true
bottom_footnotes = true
# To use a Zola built-in theme, CSP needs to allow unsafe-inline for style-src.
highlight_theme = "css"
smart_punctuation = true

After:

[markdown]
bottom_footnotes = true
smart_punctuation = true

[markdown.highlighting]
highlight_code = true
theme = "catppuccin-frappe"
style = "class"

Notes:

• highlight_code had to move into [markdown.highlighting] (0.22.0 rejects it under [markdown]; the build error itself listed the valid [markdown] fields).
• highlight_theme = "css" is replaced by style = "class" (same effect: emit CSS classes, no inline colours) plus theme = "catppuccin-frappe". This matches the tabi 0.22 demo config.toml at the currently-vendored submodule commit 5c90004.
• bottom_footnotes and smart_punctuation remain in [markdown] and continue to work.

Workflows

• .github/workflows/deploy-cloudflare-pages.yml: step Install Zola 0.21.0 -> Install Zola 0.22.0, tool: zola@0.21.0 -> tool: zola@0.22.0.
• .github/workflows/test-build.yml: shalzz/zola-deploy-action@v0.19.2 -> @v0.22.0 (the v0.22.0 tag exists and that action's tags track the Zola version; confirmed via the GitHub API). This also pulls the test build forward from the previously-lagging 0.19.2.

Verification (real local build, macOS arm64, official zola 0.22.0 binary)

• zola build with the migrated config: exit 0, Creating 8 pages (0 orphan) and 4 sections, zero warnings. (For comparison, the 0.21.0/Syntect build emitted "Highlight language not found" warnings for text, verilog, and flag; under Giallo text and verilog no longer warn.)
• Highlighting confirmed working: built HTML contains class="z-..." markup (e.g. z-keyword z-control, z-entity z-name z-function), and tabi's sass/parts/_syntax_theme.scss (compiled into main.css) defines those .z-* classes. So code blocks are styled, not left bare.
• The previously-warned pages render highlighted: blog/hitcon2025 (verilog), blog/game0x2025w1 (text), and blog/idek2025-writeup (flag) all contain z- class markup.
• KaTeX/math output present (katex.min.css emitted; katex markup in posts).

error_on_missing_language decision

Left OFF (omitted). Enabling error_on_missing_language = true was tested and turns a missing grammar into a hard build error:

ERROR Reason: Language `flag` not found for syntax highlighting in "blog/idek2025_writeup.md".

• Unsupported-by-Giallo language found: flag (used in content/blog/idek2025_writeup.md). This is a made-up fence label, not a real language.
• verilog and text, which warned under 0.21/Syntect, are accepted by Giallo and do not error.

Leaving the flag off keeps the build green. Follow-up (optional, not in this PR): change the flag fence to plain ``` (or text) and then error_on_missing_language could be enabled.

Risks for the reviewer

• Colour shift possible. Giallo themes are not byte-identical to Syntect themes, and the class taxonomy changed. Because style = "class", actual colours come from tabi's _syntax_theme.scss, not from the theme value, so this should be minimal — but it is worth a visual spot-check of a code-heavy post.
• Zola 0.22.0 also auto-emits a public/giallo.css. tabi does not link it (its templates load main.css), so it is dead weight in the output but harmless.
• test-build.yml checks out submodules non-recursively (submodules: true). tabi has a nested submodule; the deploy workflow uses submodules: recursive. This is pre-existing and unchanged here, but if the test-build job fails on a missing nested submodule that predates this PR.

Not verified

• The two GitHub Actions workflows were not executed; only the local zola 0.22.0 build was run. The v0.22.0 shalzz tag and the zola@0.22.0 install-action target were confirmed to exist but the runners' end-to-end behavior was not exercised.