fix(chat): explicit lumis theme; element-less tool body; error-panel spacing

[nhobes]

Three more petal.build finds from Nic, riding the unpublished 4.15.1:

1. Colourless code blocks - the markdown pipeline passes formatter: :html_inline and relied on lumis defaulting a theme. lumis 0.6 (playground) does; lumis 0.7 (marketing, resolved newer) does not - structure renders, tokens don't. Reproduced both ways in isolation; {:html_inline, theme: "onedark"} now travels explicitly and verifies on both lumis generations.
2. Empty padded strip in the error tool card - HEEx counts the whitespace before the first named slot as inner content, so <.tool_call> carrying only <:error_actions> renders a whitespace-only widget body past the @widget != [] guard. An element-less body now hides itself via :not(:has(*)); the slot doc says bare-text widgets need a wrapper.
3. Error band → Input panels junction - panels normally breathe off the body's bottom padding; with no body they were crammed against the red band. Ruling per Nic's question: the red stays the failure message's band, Input stays neutral evidence - they get pt-3 separation, not red envelopment.

Suite 1812/0 with three new invariants.

🤖 Generated with Claude Code

[greptile-apps]

Greptile Summary

This PR restores syntax highlighting by passing Lumis an explicit theme and refines tool-call layout for named-slot-only error cards.

• Configures the onedark theme explicitly for chat code blocks.
• Hides tool bodies containing no child elements.
• Adds spacing between error rows and subsequent tool panels.
• Documents the wrapper requirement for bare-text tool output and adds CSS-source invariants.

Confidence Score: 5/5

The PR appears safe to merge with no actionable correctness or security defects identified.

The explicit formatter configuration and CSS changes align with the documented behavior, and the investigated compatibility concerns either predate this change or are explicitly covered by the updated component contract.

Important Files Changed

Filename	Overview
lib/petal_components/chat.ex	Explicitly configures the syntax-highlighting theme and documents the element-wrapper requirement for tool body content.
assets/default.css	Hides element-less tool bodies and adds spacing where tool panels directly follow an error row.
test/petal/css_assets_test.exs	Adds source-level assertions covering the new CSS selectors and explicit Lumis formatter configuration.
CHANGELOG.md	Documents the syntax-highlighting and tool-card layout fixes.

_{Reviews (1): Last reviewed commit: "fix(chat): explicit lumis theme; element..." | Re-trigger Greptile}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.40%. Comparing base (644d13c) to head (3daf6bf).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #689   +/-   ##
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  Coverage   95.40%   95.40%           
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  Files         162      162           
  Lines        8045     8045           
=======================================
  Hits         7675     7675           
  Misses        370      370           

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