v2.28.0 — perf pass: browser freeze fixes + latency tab hardening

Browser performance (fixes freezes under streaming load)

• Narrow zustand selectors in ChatView, AppSidebar, SessionDetailView, SessionTasksKanban, UsageBar, MainShell, App — unrelated store mutations no longer re-render these components
• Batched delta writes via new recordDelta() action (4 set() per delta → 1)
• Coarse hasAnyActivity boolean subscription instead of raw lastActivity timestamp — stops per-delta ChatView re-renders
• Stable messageGroups useMemo deps + module-level EMPTY_AGENTS fallback — MessageGroup memo now actually skips re-renders during streaming
• React.memo on MessageGroup, MessageContent, ToolCard, ThinkingBlock
• Virtualization threshold lowered 500 → 80 (typical mid-sized conversations now benefit)

Latency tab correctness

• Skip effectiveTokensPerSecond when tool time dominates (>90% of gen time) — prevents absurd rates from tiny residual divisor
• finalizeRunLatency fallback to caller-provided sessionKey when markRunStart was missed
• TTL reaper for orphan runTimings (10min)
• Strict toolCallId dedup on both call and result counters — tolerates duplicate phase events
• Streaming gap threshold 1s → 3s (accommodates batched-delta providers like MiniMax)
• Gap attributed to toolTimeMs only while a tool is actually in flight
• localStorage-backed per-session latency history (7-day cap, max 200/session)
• UI: cap notice when showing last 200, tokenization estimate tooltip, corrected gap-attribution hint