fix(tips): raise the post-tip keyboard from UIKit instead of @FocusState

[bmc08gt]

Problem

After #524, completing a tip opened the chat but only focused the message field — the keyboard never came up.

Root cause

#524 set the composer's SwiftUI @FocusState = true on appear. That updates SwiftUI's focus state — the caret and the isComposing bar-morph, which is why the field looked focused — but it does not present the system keyboard. The composer lives inside a nested UIHostingController bridged through UIKit (ChatScreenRepresentable → ChatScreenViewController), and a programmatic @FocusState doesn't cross that hosting boundary. Only a real becomeFirstResponder does — which is why tapping the field (a genuine UIKit responder event) always raised the keyboard.

Every working focus-on-appear in the app (EnterPhoneScreen, LoginScreen, etc.) is a pure-SwiftUI NavigationStack/sheet screen — some raise the keyboard with only a 100 ms delay, mid-transition. So the 350 ms .task delay was never the issue; the hosting boundary was, and no amount of extra delay would have fixed it.

Fix

Raise the keyboard from the UIKit layer that already owns keyboard behavior (per ChatScreenViewController's own doc: "All scroll, keyboard, and flow-under behavior lives in the UIKit screen"):

• ChatScreenViewController.viewDidAppear — the exact moment the push transition has settled — calls becomeFirstResponder() on the composer's text field.
• Guarded by a didFocusComposer flag → strict once-per-open (tighter than the old .task, which could re-fire on re-appear).
• Removed the @FocusState/.task focus hack and its focusOnAppear plumbing through ConversationBottomBar / ConversationComposer.
• The isComposing bar-morph is preserved — when the keyboard rises, SwiftUI syncs @FocusState and the existing .onChange(of: isFocused) fires.

The .tipConversationWithKeyboard route and openKeyboard → focusOnAppear → focusesComposerOnAppear chain are unchanged; only the mechanism that acts on the flag moved from SwiftUI to UIKit.

Testing

• ./Scripts/build.sh --device → BUILD SUCCEEDED
• Installed to a physical device; verified a completed tip opens the chat with the keyboard raised.