ui: GitHub-Actions-style detail page, sub-steps, mini-tile run-view

Reshapes the detail page into a run-view: hybrid horizontal pipeline
+ expanded active-step pane with sub-steps, a per-step log pane with
line-numbered permalinks and client-side search, and a runs-history
sidebar that navigates via ?run=N. Default step is server-picked
(running → failed → Reporting) so the operator lands on the thing
that's moving.

Adds a sub_steps table + SSE topic (substep-{run}-{stage}-{ordinal})
so per-disk and per-pass work (SMART, CPUStress CPU/RAM, Storage,
GPU) is visible in the UI instead of buried in stage summary JSON.
Agent emits sub-step reports from existing per-iteration loops.

Dashboard tiles become a mini run-view with a 9-dot step strip so
the operator reads run health across the whole grid at a glance.
Register page gets the same card shell + button styling.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/model/model.go

@@ -86,6 +86,24 @@ type Stage struct {
 	SummaryJSON string
 }
 
+// SubStep is a finer-grained unit within a Stage, authored by the agent.
+// Not every stage has sub-steps; those that do (CPUStress, SMART per-disk,
+// Storage per-disk, GPU per-device) surface them so the UI can render a
+// GitHub-Actions-style collapsible list. Sub-steps share the StageState
+// enum with Stage; Ordinal is 0-based within StageName for a given RunID
+// and is how the UI and SSE events key each row.
+type SubStep struct {
+	ID          int64
+	RunID       int64
+	StageName   string
+	Ordinal     int
+	Name        string
+	State       StageState
+	StartedAt   *time.Time
+	CompletedAt *time.Time
+	SummaryJSON string
+}
+
 type Measurement struct {
 	ID      int64
 	RunID   int64