Unlock fee estimate, root loading state, global RPC-down banner (#240, #242, #248)

[richardtoms100]

Summary

• UnlockModal doesn't show transaction fee estimate #240 — `UnlockModal` submitted unlock transactions with no fee estimate — users only found out the cost once Freighter's signing prompt appeared. Added `buildUnlockAssetsTransaction`/`simulateUnlockAssets` (mirroring the existing lock-side pair) and `useUnlockAssetsFeePreview` (mirroring `useLockAssetsFeePreview`'s debounced-simulation pattern), wired into the modal's existing amount-preview box.
• No loading state on initial app boot #242 — Color-mode FOUC was already prevented by the existing `ColorModeScript` + `colorModeManager={localStorageManager}` setup, and font-load FOUT by `next/font`'s `display: "swap"` — both already correct, so no change was needed there. The one real gap: no Suspense fallback for the root segment. Added `app/loading.tsx` (Next.js's App Router convention), shown during initial boot / a slow route transition instead of a blank screen.
• No toast or banner when Soroban RPC node is unreachable #248 — When the Soroban RPC is down, every page independently showed its own query error with no indication it's a shared, app-wide outage. Added `useRpcHealth` (a lightweight `getHealth()` poll, 30s interval) and `RpcUnreachableBanner`, mounted globally in `AppShell`, mirroring `NetworkMismatchBanner`'s sticky-alert pattern.

Already resolved on inspection

• No transaction history pagination or infinite scroll #241 ("No transaction history pagination or infinite scroll") — `history/page.tsx` already has full client-side pagination: `PAGE_SIZE = 20`, `page` state, and working Prev/numbered/Next controls (confirmed rendered in the JSX, not just wired in state). The one real nuance: the underlying `getUserTransactionHistory` call still fetches the full history in one RPC event-scan and paginates client-side rather than lazily fetching per-page — worth a follow-up if history genuinely grows large for real users, but the issue's literal claim ("no pagination controls") doesn't hold. Left a comment on the issue.

Test plan

• `npx tsc --noEmit` — no new type errors.
• `npx eslint` on all touched files — clean.
• `npx vitest run src/hooks/useSorobanQuery.test.ts` — 12/12 passing.

Closes #240
Closes #242
Closes #248
Closes #241

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