fix: brighten dark-mode section headers and bold markdown

[philmerrell]

What changed

Two dark-mode text-color tweaks. Light mode is unchanged.

Sidebar date headers (session-list.html): the Today / Yesterday / Last 7 Days labels went from dark:text-gray-500 to dark:text-gray-300. They had been one step darker than the session titles beneath them (gray-400), so they were the least visible text in the list. Now they sit one step lighter.

Bold in assistant responses (styles.css, the dark .message-block block):

Dark mode	Before	After
Response prose (.message-block.text-block)	gray-200 (inherited from the bubble)	gray-300
strong	gray-200, weight only	white
h1–h6	gray-200	white
a strong	link color	link color (inherit)

Why

Bold only changed weight in dark mode, and a 400→600 weight step barely shows on a dark surface. Making bold white alone wasn't enough, because the bubble's gray-200 body is already nearly white. So the prose steps down one notch and bold/headings step up to white. Headings are included so the prose change doesn't dim them.

Notes for review

• The prose change is scoped to .text-block (markdown text blocks). Tool rails, status lines and the rest of the assistant bubble keep the wrapper's gray-200.
• gray-300 on the gray-900 bubble is still roughly 12:1 contrast, well above AA.
• .message-block is also used by the announcement preview and the skill detail page. Those don't carry .text-block, so only the strong/heading brightening reaches them. Their bodies are gray-200/gray-300, so white bold still reads as a step up.
• Verified in a real conversation against a local app-api: computed colors are prose gray-300, strong and h2 white, header gray-300 over title gray-400.

🤖 Generated with Claude Code