Improve chat loading and Markdown readability

[zcmx]

Problem

Two presentation issues reduce readability on phones:

• the session loading indicator is drawn transparently over existing chat content, so its text and spinner can blend into messages;
• Markdown headings and nested lists lose much of their source hierarchy in the compact mobile layout, especially when list items wrap.

Changes

• Give the chat loading overlay an opaque themed background and a stable test tag.
• Use existing OpenCode Markdown theme tokens for headings, strong text, bullets, and ordered-list markers.
• Add compact spacing before headings and between top-level list groups.
• Render list markers in a fixed-width column so wrapped lines align with item text.
• Preserve and render nested list indentation without introducing cards or large article-style spacing.
• Add parser coverage for nested list indentation and retain source numbering for ordered lists.

Mobile-space tradeoff

The layout intentionally uses small 2–8dp gaps rather than copying desktop/TUI spacing literally. It sacrifices a small number of visible pixels to make hierarchy scannable while keeping the chat dense.

Verification

• Manually verified on a Samsung SM-A546E with the hardened build.
• StreamingMarkdownParserTest passes on the integrated branch.
• :app:assembleHardened succeeds on the integrated branch.
• git diff --check passes.

The clean main branch currently blocks local Android unit-test compilation because compileDebugJavaWithJavac targets JVM 17 while compileDebugKotlin targets JVM 21. The same parser test and hardened build passed before extracting these commits onto the clean PR branch.