Use UI flag createdAt as announcement start date via /ui-flags/v2 FEDEV-3937

[qwinndev]

Frontend half of FEDEV-3937. Reads UI flags from the keeper's new /ui-flags/v2 endpoint and uses each flag's activation time (createdAt) as the start date for flag-driven announcements that have no explicit startDate — so the Announcements page and the What's New toast display and sort by the real go-live date instead of falling back to the removal date (endDate).

Stacked on whats-new-toast-announcements-page-fedev-3855 — this PR is based on that branch; retarget to release once it merges. Keeper side: gmx-io/gmx-synthetics-keeper#529.

Changes

• fetchUiFlags now hits /ui-flags/v2; UiFlags is Record<string, { enabled, createdAt, updatedAt }>. Every consumer reads ?.enabled — no runtime behavior change.
• New getEventStartDate / updated getEventSortDate, with precedence: explicit startDate > flag createdAt > endDate. Used by the Announcements page sort + card and the What's New toast.
• Cross-chain flag merge picks the earliest createdAt among enabled chains.
• Bumped the UI-flags localStorage cache key to invalidate old boolean-shaped entries.
• Added unit tests for the start-date helpers.

https://linear.app/gmx-io/issue/FEDEV-3937

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