test(frontend): cover max balance button behavior

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Motivation

Recent MaxBalanceButton changes added fee-aware max calculations and a maxAmount cap used by Liquidium repayment/supply flows. The existing spec only checked rendered labels, leaving the user-funds path where clicking Max mutates amount and re-applies after fee changes untested.

Risky behavior now covered:

• Native-token fees are reserved before comparing against a higher maxAmount cap.
• Lower maxAmount caps still win after fee reservation.
• Clicking Max writes the capped amount and marks amountSetToMax.
• Zero balances remain a no-op.
• Fee changes debounce and re-apply Max when the amount was already set to Max.

Changes

• Added MaxBalanceButtonTestHost.svelte to expose bindable amount and amountSetToMax state in component tests.
• Extended MaxBalanceButton.spec.ts with focused fee/cap/click/debounce regression cases.

These tests materially reduce regression risk because MaxBalanceButton is shared across send/stake/Liquidium flows and directly controls the maximum spendable amount shown and submitted by users.

Tests

• npx @sveltejs/mcp svelte-autofixer ./src/frontend/src/tests/lib/components/common/MaxBalanceButtonTestHost.svelte --svelte-version 5
• npx prettier --check src/frontend/src/tests/lib/components/common/MaxBalanceButton.spec.ts src/frontend/src/tests/lib/components/common/MaxBalanceButtonTestHost.svelte
• npx eslint src/frontend/src/tests/lib/components/common/MaxBalanceButton.spec.ts src/frontend/src/tests/lib/components/common/MaxBalanceButtonTestHost.svelte --max-warnings 0
• npx vitest run src/frontend/src/tests/lib/components/common/MaxBalanceButton.spec.ts

Notes:

• npm run test -- src/frontend/src/tests/lib/components/common/MaxBalanceButton.spec.ts is currently blocked before Vitest by an existing unrelated TypeScript error in src/frontend/src/sol/utils/sol-instructions-system.utils.ts (SystemInstruction.CreateAccountAllowPrefund not assignable to never).
• npm run check:tests is blocked by the same existing unrelated TypeScript error.
• npm ci is blocked on this checkout by an existing package-lock sync issue (@emnapi/core / @emnapi/runtime missing from lockfile), so dependencies were installed with npm install --package-lock=false to keep this PR test-only.