feat(server): add fallback URL with automatic failover

[yashpatil27]

Closes #14

Summary

Adds a fallback endpoint to each server. When the primary URL is unreachable, the app automatically tries the configured fallback (LAN ↔ WAN, DDNS vs static IP, reverse proxy vs direct, etc.), so the user doesn't have to manually edit the server every time the network context changes.

The feature is fully optional — existing servers continue to work exactly as before with no fallback configured, no migration required.

Behavior

Save / Edit

• New "FALLBACK URL" section in the Add Server and Edit Server screens, between Server Info and Authentication.
• Toggle "Use fallback URL". When enabled, the section reveals fallback host, fallback port, and a fallback HTTPS switch.
• Authentication credentials are shared between primary and fallback (fallback is treated as an alternate route to the same qBittorrent instance).
• Save validates: fallback host required when enabled, fallback port must be 1–65535 when provided.
• Edit screen loads and round-trips every fallback field.

Connect

• The connection flow attempts the primary endpoint first.
• On a network error (timeout, ECONNABORTED, ERR_NETWORK, 5xx, etc.) the fallback is tried next.
• On an authentication / permission failure the fallback is not tried because credentials are shared — failing again would just waste time.
• Whichever endpoint connects becomes the active one for the session. The stored current_server_id is the same regardless of which endpoint won, so the rest of the app sees a single logical server.
• reconnect() and checkAndReconnect() reuse the same flow — fallback happens automatically on stale-connection recovery.

Test Connection

• Primary-only servers: unchanged toast ("Connection test successful!" or error message).
• Fallback-enabled servers: both endpoints are tested and a compact per-endpoint toast is shown, e.g. "Primary: OK · Fallback: OK" or "Primary: failed · Fallback: OK".

Settings tab

• When a server with fallback is connected, the connection card shows a small subtitle: "Connected via primary" or "Connected via fallback".
• Servers without fallback render exactly as before.

What changed

Data model — types/api.ts

• ServerConfig extended with optional fields: useFallback, fallbackHost, fallbackPort, fallbackUseHttps, fallbackBasePath (last one is reserved for a future UI; data-model parity only).
• New ServerEndpointKind = 'primary' | 'fallback' union.

Helpers — utils/server.ts

• hasFallback(server) — true when fallback is enabled and a host is set.
• resolveServerEndpoint(server, endpoint) — returns a transient ServerConfig whose host/port/useHttps/basePath are swapped for the chosen endpoint, leaving id/credentials intact so cookies and stored IDs remain coherent.
• getServerEndpointLabel(server, endpoint) — display string helper.
• getActiveEndpoint(server, activeServer) — derives which endpoint the API client is currently on by comparing host/port/HTTPS against both resolved endpoints.

Connection logic — services/server-manager.ts

• connectToServer(server) refactored: tries primary via a shared private connectToEndpoint(server, resolved, endpoint) path, catches network errors, and re-runs the same path against the resolved fallback when configured. Non-network errors propagate unchanged. The most informative error (fallback's, when both fail) is surfaced.
• testConnection(server, signal) returns the new ConnectionTestResult shape (extends the old { success, error }) with optional primary/fallback keys when fallback is enabled. A new internal testEndpoint runs the per-endpoint check and translates auth/network errors into the consistent shape. Cancellation propagates between the two endpoint tests.
• New exported types: EndpointTestResult, ConnectionTestResult.

Context — context/ServerContext.tsx

• New state: activeEndpoint: 'primary' | 'fallback' | null.
• refreshActiveEndpoint(server, connected) derives the active endpoint from apiClient.getServer() and is called after every successful connect/reconnect/auto-connect path. Cleared on disconnect and on connection errors.

Storage — services/storage.ts

• saveServer() persists useFallback, fallbackHost (with stripProtocol), fallbackPort, fallbackUseHttps, and fallbackBasePath.
• getServers() strips any stray protocol on fallbackHost for legacy safety.
• No migration is needed; missing fields naturally mean fallback is disabled.

UI

• app/server/add.tsx: new section, new state, validation in both Save and Test paths, save includes fallback fields, per-endpoint test-result toast.
• app/server/[id].tsx: same UI section, plus loads/round-trips all fallback fields and preserves fallbackBasePath even though the UI does not surface it.
• app/(tabs)/settings.tsx: imports hasFallback, consumes activeEndpoint, shows the connected-via subtitle when applicable.

i18n — locales/{en,es,zh,fr,de,ru}/translation.json

12 new keys per locale:

• server.fallbackUrl, server.useFallback, server.useFallbackHint, server.fallbackUseHttps
• server.endpointPrimary, server.endpointFallback
• server.testEndpointOk, server.testEndpointFail
• server.connectedViaPrimary, server.connectedViaFallback
• placeholders.fallbackHost
• errors.fillFallbackHost

Backward compatibility

• Existing servers stored in AsyncStorage have no fallback fields → useFallback reads as undefined → hasFallback() returns false → all flows behave exactly like before.
• The legacy theme / other preferences are unaffected (this PR only touches server-related types and code).
• Saved password remains in SecureStore as before.

Tested

• Existing primary-only server: connects, edits, tests as before.
• Fallback enabled and primary reachable → connects via primary; settings subtitle "Connected via primary".
• Primary unreachable, fallback reachable → connects via fallback; subtitle "Connected via fallback".
• Both unreachable → meaningful error.
• Auth failure on primary → does not attempt fallback.
• Add Server validation: missing fallback host when enabled is blocked; invalid fallback port is blocked.
• Edit Server: fallback fields load and round-trip correctly.
• Test Connection toast: per-endpoint outcomes display correctly.
• npx tsc --noEmit passes clean.

Out of scope (intentionally deferred)

• Mid-session failover (switching endpoints during an already-connected session).
• More than one fallback endpoint.
• Per-endpoint credentials.
• UI for fallbackBasePath (field exists for data-model parity).