feat(chat): send cash into a group chat as a cash link

[bmc08gt]

Group chats hid the $ button because a group has no single participant to pay. This lets a group send cash as a cash link: the app funds the link and posts its bare URL into the group as an ordinary text message. LinkCardClassifier already turns a link-only message into a LinkCard.Cash, so the transcript shows the card with the amount and no text bubble. The first member to tap it claims the funds.

Brief: code-payments/flipcash-client-orchestrator#22.

What changes

• Send path. ChatCashLinks.sendToChat is a new entry point in :shared:session. It creates a gift card, funds it, then posts Linkify.cashLink(entropy) through ChatCoordinator.sendMessage. Nothing is posted unless funding succeeds. If the post fails, it cancels the remote send so the funds return, and reports the failure. It runs under NonCancellable, so leaving the chat mid-send can't strand a funded link that was never posted.
• Shared funding. Funding and cancelling move out of GiftCardSharingDelegate into GiftCardFunding, so the bill's "Send as Link" and the group send use one path. The bill path keeps its own error UI and bill dismissal.
• Chat. OnSendCash now lets a group through and skips the tip-DM fee branch. The null participant arm of OnSendRequested calls sendToChat, then uses the existing send state, SendComplete, and error handling. ChatBottomBar drops the group check.
• Analytics. Sent Cash has no property for where the cash went, so a group send reports Send Cash Link with a new Cash Link Choice value, Posted to group chat. It reports Failure too, which the drift note in events.toml now records. EVENTS.md is regenerated.

No proto change: the message is plain text, and group CashContent stays reserved.

Out of scope: ProfileShortcuts.kt's commented-out onSendCash and the orphaned GiftCardFundingWorker.

Tests

• ChatCashLinkDelegateTest: funds then posts exactly the URL; a failed post cancels the gift card; a failed fund posts nothing.
• ChatGroupCashLinkTest: a group OnSendCash opens amount entry; a group send goes to ChatCashLinks and reports Send Cash Link; contact and tip sends still go to their delegates.
• TransferEventsTest: the new Cash Link Choice value.