style(markdown): visible gridlines on rendered tables

[probertson]

Rendered Markdown tables now have visible gridlines in every reading view.

Before: the ⌘M chat view had no table styling at all, so cells ran together with no lines. The ⌘M output view and the editor's Markdown preview drew lines with --border, a 10% separator that made them hard to see.

Now (one rule group for both containers, .term-md__content and .term-chat__text):

• 1px gridlines at 22% of the text colour (follows light/dark).
• A lightly tinted, semibold header row.
• A table wider than the view scrolls sideways inside itself, so the chat thread never does.
• Column alignment from Markdown (---:, :---:) still applies. It arrives as an align attribute, which an author text-align would override, so only headers without one are start-aligned (to match their cells instead of the browser's centred default).

The chat view's plan and question cards render through the same container, so their tables get the same treatment. Sticky notes are unchanged.

How it was checked

• New styles.markdown-table.test.ts pins the rules for both containers and that cells set no text-align. The src/renderer/styles suites pass.
• In an isolated dev build: asked Claude for a three-column table with a right-aligned column and opened the chat view. Computed styles showed the 22% border, header tint, display: block with overflow, and the right-aligned column kept its alignment. Checked a screenshot before and after the header-alignment tweak.
• Manually on macOS: works as expected.

Not checked: the light theme and the output view/editor preview by hand (same rules, covered by the test).

🤖 Generated with Claude Code