Decompose App.tsx phase 2 step 3: extract useConnectionLifecycle

[cliffhall]

Step 3 of phase 2 of the App.tsx decomposition — see #2129 for the phase, #2126 for the whole effort.

This is a stacked PR. Its branch is cut from step 2's and its PR targets that branch, not v2/main. Do not land it before step 2.

Scope — useConnectionLifecycle(session, oauth, stores), ~700 lines

• latencyMs, connectStartRef, connectErrorMessage, recordConnectError
• clearOAuthResumeOnExplicitDisconnect, finalizeExplicitDisconnect, resetSessionScopedUiState
• the connection effects
• setupClientForServer (~215 lines, the heaviest single function in the file)
• the ~250-line effect that drives it
• onToggleConnection
• onDisconnect, its effect, onReauthenticateFromBanner

Supplies the onReconnect callback step 2 injects.

Constraints carried from #2129

• resetSessionScopedUiState reaches into most of phase 1's UI state to clear it on disconnect. Give it a narrow reset() surface from each phase 1 hook rather than handing it every setter — otherwise this hook depends on everything and nothing was decoupled.
• No behavior change.

Done when

• useConnectionLifecycle is its own hook
• ≥90% on all four coverage dimensions
• npm run ci green, all three web smokes included