feat(examples): task form layout recipe (#44)

[IgorShevchik]

Summary

• Added FormTaskFormExample.vue — Bitrix24-style task form built exclusively from standard b24ui components, shown in both nuxt and demo playgrounds under /components/form
• Added ### Task form layout section to docs/content/docs/2.components/form.md with ::component-example + ::prompt block
• Added skills/b24-ui-nuxt/references/recipes/task-form.md — full reference recipe with component tree, key patterns, and common mistakes

What the layout demonstrates

Single-column composition using only standard b24ui components — no custom CSS beyond component props:

Block	Components
Task title	B24Input size="xl" no-border
Description area	B24Editor (markdown) + B24EditorToolbar (mention, lists) + attach/expand buttons
Responsible persons	B24Card b24ui.body='p-0' + divide-y rows + B24Avatar + B24InputDate + B24Popover+B24Calendar for date picker
Watchers	B24Card with #header slot + B24Avatar
Action sections	17 × B24Button with active/inactive color states (air-secondary-accent-2 / air-secondary-no-accent) + aria-pressed
Form actions	B24Button air-primary (Save) + B24Button air-tertiary (Cancel)

Test plan

• Open /components/form in demo playground — task form section renders correctly
• Open /components/form in nuxt playground — same
• Verify B24InputDate deadline field opens calendar picker via B24Popover
• Verify active action buttons are visually distinct from inactive ones
• Verify action buttons have correct aria-pressed attribute
• Check form.md in docs — task form section renders with example and prompt block

Closes #44

[IgorShevchik]

Review note (cross-PR consistency) — there's a problem with the examples navigation that needs to be resolved before this can merge. Flagging it on both this PR and #46, since they collide.

1. Guaranteed merge conflict with #46. Both this PR and #46 add an examples const and a { id: 'examples', label: 'Examples' } group to the same lines of playgrounds/nuxt/app/composables/useNavigation.ts and playgrounds/demo/app/composables/useNavigation.ts (plus an icon import at the same spot). Whichever lands second will hit a git conflict in four files.

2. The two PRs implement the group inconsistently. This is the part to settle:

• This PR (feat(examples): task form layout recipe (#44) #47): route to: /examples/${example}, new pages/examples/task-form.vue directory.
• feat(examples): Chat/Channel Detail Panel layout example (#45) #46: route to: /components/${example}, page stays under pages/components/channel-panel.vue.

So the same nav group would point into two different route trees depending on which item you click. The current repo practice is that every playground page lives under pages/components/* and routes to /components/* (there is no pages/examples/ tree today). A composed "example" is still surfaced through the relevant component docs page via ::component-example (your form.md change does this correctly).

3. Recipe not registered in skills/index.json. This PR adds skills/b24-ui-nuxt/references/recipes/task-form.md but does not add it to skills/index.json. That file enumerates skill files explicitly, so the recipe won't be picked up. Compare with #46, which correctly registers references/recipes/detail-panel.md in skills/index.json — that's the pattern to follow.

Suggested resolution: agree on one convention with #46 (route tree + whether a top-level examples group is even introduced now), land one of them first, then rebase the other on top. Also add the recipe to skills/index.json.

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