Create and edit modal polish (#73)

[SlyDave]

Summary

• Rename the board add control to Create task to match the modal title.
• Description spans full modal width; colour picker always visible with None default (no checkbox); wide colour preview.
• Remove Optional/Required helper text; disable Create task when description is empty; enforce title/description maxlength via domain constants.

Closes #73

Acceptance criteria

• Create task label on primary add affordance — BOARD_ADD_TASK_LABEL.
• Description full width — w-full on description field and form.
• Colour picker always visible, default none, no checkbox, wide preview — preset select + full-width colour input.
• No Optional/Required helpers — hints removed; description still marked required with asterisk.
• Create task disabled without description / enabled with non-empty — taskFormCanSubmit + :disabled.
• Title max 50 / description max 5000 — TITLE_MAX_LENGTH / DESCRIPTION_MAX_LENGTH on inputs.

Test plan

• Open create modal: Create task disabled until description entered
• Confirm no Optional/Required helper text; description shows required asterisk
• Colour preset defaults to None; picker visible and wide; custom colour submits a hex
• Title/description cannot exceed 50/5000 characters

[SlyDave]

QA verdict: approved

Verified against #73 acceptance criteria on frontend/issue-73 (local Nuxt at :3073) plus unit tests (taskFormCopy.test.ts, boardCopy.test.ts).

Criterion	Result	Evidence
Primary add label Create task	Pass	[data-testid=board-add-task] text
Description full modal width	Pass	Description width matches form (441px)
Colour picker always visible; default None; no checkbox; wide preview	Pass	Colour input visible (~266px); preset None; no checkbox
No Optional/Required helper text; required asterisk/attr on description	Pass	Form copy clean; description has required
Create task disabled when description empty	Pass	Submit disabled until description filled
Create task enabled with non-empty description	Pass	Enabled after fill
Title max 50 / description max 5000	Pass	maxlength 50 / 5000

CI green on the PR.