feat: Enhance AdminPanel with burn_admin, pending admin transfer & vesting management

[jayteemoney]

Overview

Enhances the token AdminPanel with three previously-missing admin capabilities, plus the one read-only contract getter needed to support them.

Closes #300
Closes #301
Closes #302

#300 — Expose permanent admin burn (burn_admin)

The Clawback card is now a Remove Assets card with a Confiscate/Destroy toggle:

• Confiscate to Admin → clawback(from, amount) (reversible; tokens move to admin balance).
• Permanently Burn → burn_admin(from, amount) (irreversible; reduces total supply, increments total_burned).

Copy clearly differentiates the two paths. Shared form, decimal-scaling, and preflight simulation are reused. No contract change required (burn_admin already existed).

#301 — pending_admin() getter + two-step transfer UI

• Contract: added pub fn pending_admin(env: Env) -> Option<Address> (reads DataKey::PendingAdmin) with a unit test.
• UI: fetches the pending admin on mount; renders a "Admin transfer pending → <address>" banner; shows Accept Admin Role only when the connected wallet matches the pending address; gives the current admin a Cancel Pending Transfer action (overwrites the proposal via propose_admin). State is re-read after every propose/accept/cancel.

#302 — extend_cliff / revoke vesting UI

New Manage Vesting card (vesting contract + recipient + optional schedule index):

• Extend Cliff → extend_cliff(recipient, new_cliff_ledger, Some(index)), computing new_cliff_ledger from getLatestLedger() with the same days * 17280 math as create_schedule.
• Revoke → revoke(recipient, Some(index)) behind an explicit confirmation explaining that unvested tokens return to the admin.

Option<u32> index is encoded correctly (Void = None, value = Some).

Verification

• cargo test -p soroban-token — all tests pass (incl. new pending_admin test)
• tsc --noEmit — clean
• eslint — clean

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