feat(kanban): add optional task description preview

[totondruppusu]

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What this PR does

Adds an option to show a brief preview of the description of tasks in board view.

Linked issue

Feature as discussed in issue #59

Checklist

• I discussed this change in an issue and got a thumbs-up before writing code.
• npm run build passes with zero errors.
• npx eslint src/ passes with zero warnings — includes no inline styles, sentence-case UI text, no unnecessary non-null assertions.
• New UI follows the Quiet Architect design system (no emojis, no hard corners, no 1px dividers, no pure black text, depth via tonal stacking).
• New modals are created through ModalFactory, never instantiated directly.
• No element.style.* assignments — CSS classes only.
• No dynamic await import() — static imports only.
• Dates use YYYY-MM-DD format; UTC operations only (T00:00:00Z, setUTCDate()).
• This PR is focused on one logical change. Unrelated cleanup is in a separate PR.

[StepanKropachev]

Thanks for the contribution — the implementation follows the kanbanShowSubtasks precedent cleanly, scope matches what we agreed in #59 (default off, 3-line clamp, plain text), and the CHANGELOG entry is in the right spot.

A couple of things to address before I can merge:

1. Merge conflict with main — please rebase. GitHub is showing the branch as conflicting.
2. Trailing newline on styles.css — the diff strips the final newline at EOF. oxfmt requires a trailing newline, so pnpm check will fail on this. Running pnpm fix should restore it.

After rebasing, please confirm pnpm check && pnpm check:submission && pnpm test all pass locally — same gate CI runs.

Minor (not blocking): var(--pm-text-muted) has no fallback, but that's consistent with the rest of styles.css and fine since the card never renders inside a modal.

Looks great otherwise — thanks again!