feat(tui): support configurable cursor style, blink, and color

[fanyu]

What does this PR do?

Fixes #11305.

It adds configurable TUI cursor settings in opencode.json:

• tui.cursor_style (block | line | underline)
• tui.cursor_blink (boolean)
• tui.cursor_color (hex like #FF5733 or theme key like primary)

It applies these settings to all TUI textareas I found in opencode:

• main prompt
• question custom answer
• permission reject
• dialog prompt
• export options filename

I also added a small shared helper (resolveTextareaCursor) to keep behavior consistent and updated config.mdx + tui.mdx docs.

How did you verify your code works?

• Verified schema changes in src/config/config.ts for all three new fields.
• Verified all target textarea call sites now pass both cursorStyle and resolved cursorColor.
• Checked default fallbacks stay the same where they previously differed (theme.primary vs theme.text).
• I could not run local typecheck/tests here because bun is not available in this environment.

[github-actions]

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[fanyu]

Updated ✅ I have revised the PR description to match the required template sections:\n\n- ### What does this PR do?\n- ### How did you verify your code works?\n\nPlease re-check compliance.

[fanyu]

I shortened and rewrote the PR description using the exact template headings. It now includes only:\n\n- ### What does this PR do?\n- ### How did you verify your code works?\n\nCould the compliance check please rerun? If this still fails, I may need maintainer help to confirm the checker rules.

[fanyu]

Update: typecheck is now passing after the cursor config type fixes.\n\nCurrent failures are e2e on both linux/windows:\n- e2e/prompt/prompt-slash-terminal.spec.ts › /terminal toggles the terminal panel\n- e2e/settings/settings-keybinds.spec.ts › changing terminal toggle keybind works\n\nThese failures are in app terminal-toggle behavior; this PR only touches TUI cursor config + docs in and config schema/docs.\n\nIf needed, I can rebase again and/or help bisect, but this looks unrelated to the cursor-style change scope.

[github-actions]

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