feat: dashboard SSE streaming progress (#1007)

[theonlychant]

Summary

Surfaces the existing SSE event stream on the Triage Dashboard during long-running inbox scans, giving users real-time step-by-step feedback instead of an indeterminate spinner.

Why

Refresh and pre-scan runs against a busy inbox can take tens of seconds. With no per-step feedback the UI reads as broken. The agent already emits tool_start, tool_result, status, and cancelled events through SSEOutputHandler — this PR wires those events to three new dashboard components so the user can see what the agent is doing, watch cards stream in as they're classified, and cancel a run mid-flight if needed.

Linked issue

Closes #1015

Changes

• Add useSSEProgress hook subscribes to the dashboard's existing SSE connection and fans state out to consumers; no new event types introduced
• Add ProgressStrip shows current step in human-readable terms (tool_start → label map with fallback); displays count/percentage when the event carries pagination metadata
• Add StreamingCards animates classified cards into the Surfaced column one-by-one as each tool_result arrives, instead of all-at-once at the end
• Add CancelButton visible while a run is in-flight; POSTs to /api/cancel and clears on the existing cancelled SSE event
• Add tool-name → human label mapping (pre_scan_inbox_impl, draft_reply, fetch_calendar; generic fallback for any other tool)
• Wire per-tool latency timestamps into the existing perf hook (coordinates with feat(agents): multi-model parallelism — small classifier + large writer via Lemonade multi-model loading #1000 sub-issue 8 - no double-instrumentation)
• SSEOutputHandler is unchanged

Test plan

• python util/lint.py --all passes
• pytest tests/unit/ passes
• Trigger a dashboard refresh against a 25-message inbox confirm ProgressStrip updates step label on each tool_start and shows a message count during classification
• Confirm cards stream into the Surfaced column incrementally (visible one-by-one, not all at once)
• Click Cancel mid-run confirm the run stops within ~1 s and partial results remain visible
• Trigger a draft or calendar refresh confirm the strip and cancel button work for those tools too (not just pre_scan_inbox)
• Confirm no new SSE event types are emitted (event log should show only tool_start, tool_result, status, thinking, cancelled)