test(blocklist): UI tests for block/unblock flows

[bmc08gt]

Adds XCUITest coverage for the Blocking feature merged in #539.

What's covered

• BlockUnblockSmokeTests — the full round-trip: block a tip-DM counterpart from their profile → assert the conversation disappears from the Tips list → unblock from Settings → My Account → Blocked → assert the block is cleared.
• BlockedUsersSmokeTests — Settings → My Account → Blocked reaches the list and it loads (flag on).
• BlockedUsersGatingRegressionTests — the "Blocked" row is absent in My Account when the enableBlocking flag is off.

Design notes

• Non-mutating fixture. The Block affordance only exists on a tip DM, so the round-trip drives whatever standing tip DM the FLIPCASH_UI_TEST_ACCESS_KEY account already has — it never sends a tip, and captures the counterpart's display name dynamically (nothing hardcoded). With no tip DM in the feed it XCTSkips rather than fabricating one.
• Self-restoring. The block/unblock round-trip returns the blocklist to its starting state. tearDown re-runs the unblock best-effort (relaunch → login → Settings → Blocked) if an assertion fails after the block lands, so a broken run never leaves the shared account with someone blocked.
• Deterministic gating. Under --ui-testing, applyLaunchArgumentOverrides() resets all beta flags, so ["enableTips","enableBlocking"] / [] force the flags on/off regardless of any cached state.
• Adds TipsUIScreen and BlockedUsersUIScreen page objects and a blockedRow accessor on SettingsUIScreen. Test-target only — no production changes, no pbxproj edits (synchronized target).

Test plan

• TEST BUILD SUCCEEDED for the FlipcashUITests target.
• Run once the access key is configured: ./Scripts/test.sh FlipcashUITests/BlockUnblockSmokeTests FlipcashUITests/BlockedUsersSmokeTests FlipcashUITests/BlockedUsersGatingRegressionTests