Add `field.timezone()` with a curated IANA list

[akladekouassi]

Summary

Add field.timezone(label) — a specialized select with a curated IANA timezone list, current-offset display, city grouping, and a "use my timezone" button.

Why this would help

• Scheduling, calendar settings, profile preferences, automation triggers.
• Every app reimplements the timezone list; having one canonical, opinionated list shipped with runilib avoids the drift between projects.
• Live offset display ("Europe/Paris · UTC+02:00") makes selection less ambiguous.

Proposed API

field.timezone('Timezone')
  .format('iana')                    // 'iana' | 'offset'
  .preferred(['Europe/Paris', 'America/New_York'])
  .useMyTimezoneButton(true);

Suggested implementation areas

• src/core/field-descriptors/timezone/TimezoneFieldBuilder.ts (new) — built on top of SelectFieldBuilder
• src/core/field-descriptors/timezone/data.ts — curated IANA list (ship a sensible subset)
• src/core/field-descriptors/field.ts
• src/renderers/web/ — offset live-computed via Intl.DateTimeFormat
• src/renderers/native/ — same behavior, native picker styling

Acceptance Criteria

• field.timezone() returns IANA strings ('Europe/Paris') by default
• format('offset') returns 'UTC+02:00'-style values
• Live offset computed from Intl reflects DST correctly
• "Use my timezone" populates from Intl.DateTimeFormat().resolvedOptions().timeZone
• Tests covering DST samples and the format toggle

[github-actions]

Mirrored to monorepo issue: https://github.com/runilib/runilib/issues/181