fix(contract): remove redundant TotalByAsset storage key

[precious-akpan]

Summary

• RecipientTotal(Address, Address) and TotalByAsset(Address, Address) were both keyed by (recipient, asset) and updated with the exact same increment/decrement in support()/withdraw().
• Every support() call wrote two persistent ledger entries (plus two extend_ttl calls) holding the same value, doubling storage rent/fees for zero informational benefit.
• Removed the TotalByAsset DataKey variant and its writes in support().
• withdraw()'s existence/balance check and decrement now operate on RecipientTotal directly (previously used TotalByAsset for this).
• get_total_by_asset() is now a thin wrapper over get_recipient_total() — public behavior is unchanged, and this matches contract/ABI.md's existing description of get_recipient_total as "mirror[ing] get_total_by_asset and read[ing] from the same RecipientTotal storage key."
• Updated one test that poked DataKey::TotalByAsset directly in storage to use DataKey::RecipientTotal instead, since that's now the key withdraw() reads.

Closes #1038

Also fixed: 3 pre-existing test failures unrelated to #1038, blocking this PR's CI

main's Contract CI has been red since #1040's reentrancy fix merged (moves the external balance() check to the very start of withdraw(), ahead of the RecipientNotFound/ZeroBalance/WithdrawAmountExceedsBalance checks — correct per #1040's design, but it left over_withdraw, withdraw_with_no_balance, and withdraw_again_after_full_withdrawal_... asserting the wrong error, since their scenarios also happen to leave the contract's real token balance below the requested amount). Added a second commit that funds the contract directly via the token admin (bypassing support()) in each of those 3 tests, so the external balance check passes and the intended application-level check is what actually gets exercised. Confirmed this reproduces identically on unmodified main (i.e., it's not something my #1038 change introduced).

Test plan

• cargo test (contract/contracts/support_page): 27/27 passed (previously 24 passed / 3 pre-existing failures — see above).
• cargo clippy -p support_page --all-targets — no new warnings introduced (2 pre-existing len_zero warnings, untouched by this diff).
• Regenerated test snapshots confirm the [Contract] RecipientTotal and TotalByAsset are now fully redundant, doubling storage writes and rent per call #1038 fix: each now has one fewer contract_data ledger entry (the removed TotalByAsset read/write) per support()/withdraw() call.

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