feat(messenger): rebuild another user's profile with a pinned start-chatting action

[bmc08gt]

Third slice of the profile refresh, matching code-payments/code-ios-app#987: another user's profile is rebuilt on the shared header from #1691 and owns its own chat, block, fee and payment state. Stacked on #1691.

Screen (features/messenger, Figma node 10990:494)

• Cover, avatar, name, handle, bio and the Minimum to Chat / Date Joined card come from the shared ProfileHeader and ProfileStatsCard.
• The action row holds Blocked and Muted chips beside Share. Share opens ProfileShareSheet without the profile card row.
• The ⋯ menu has Mute Notifications, Report and Block or Unblock, styled like the chat screen's overflow menu. It is hidden on your own profile.
• The Message and Send Cash shortcuts are gone.
• One pinned button reads "Send $X to Start Chatting", "Open Chat" or "Unblock", with the encryption note above it when the DM is encrypted. Content scrolls under it, and nothing is pinned until the full profile has loaded.
• The pinned area is ProfilePinnedActionBar in shared/common-ui/.../profile, so the group profile can pin its own button. It takes an optional slot above the button and an optional secondary text action below it.
• Toasts on this screen sit above the pinned button. FloatingToastHostState gains a per-screen bottom clearance for that (ToastBottomClearance), taking the largest clearance any open screen asks for. The bar registers its own height while it is shown.

State (ChatProfileViewModel)

• A DM exists when the generated tip chat id has members. Blocked comes from the new BlocklistCoordinator.observeIsBlocked, and blocked wins over DM-exists because a block hides the DM without removing its members.
• blockUser now writes the local blocklist row, so the chip and the button change without waiting for a sync.
• The fee is startChattingFee. When it is unknown the button opens the DM with Send Cash, as before; slice 5 replaces that path when the paid-DM gate leaves the chat.
• The view model is shared across one chat flow. Opening a different person resets everything about the last one; opening the same person again keeps the loaded profile on screen and refreshes it in the background.

Paying to start a chat

• The fee sheet (StartChattingSheet) opens inside the profile rather than as a route, so the push to the new DM happens while the profile is still on top.
• The balance, add-money, discover-currencies and send-limit checks moved from ChatViewModel into StartChattingPayer, which both screens use for their alerts. ChatViewModel's own fee payment still sends directly and goes away in slice 5.
• The sheet stays up and can't be dismissed (swipe, scrim or back) from the slide until the new DM's members arrive; then it closes and the DM opens. If they haven't arrived after 10s, as on iOS, the sheet closes and the profile stays, with the button showing "Open Chat" and a spinner until they do.
• A person who has been paid cannot be paid again while that wait runs. A failed payment unlocks the sheet, shows the error, and can be retried.
• ChatInitPaymentSheet now takes a Flow<Unit> for send completion instead of ChatViewModel's event flow, so the profile can use it.

Tests

• ProfileActionsTest covers the pinned-action and menu decisions, DefaultStartChattingPayerTest the balance, send-limit and no-account branches, and ChatProfileViewModelTest repeat opens, person switches, double payment and payment failures.
• BlockedUserDaoTest runs observeIsBlocked against Room.
• The shortcut tests are removed with the shortcuts, and the screenshot tests of the old header are removed; those had no recorded goldens.