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/db/migrations/0004_add_sub_steps.sql

@@ -0,0 +1,13 @@
+CREATE TABLE IF NOT EXISTS sub_steps (
+    id            INTEGER PRIMARY KEY AUTOINCREMENT,
+    run_id        INTEGER NOT NULL REFERENCES runs(id) ON DELETE CASCADE,
+    stage_name    TEXT    NOT NULL,
+    ordinal       INTEGER NOT NULL,
+    name          TEXT    NOT NULL,
+    state         TEXT    NOT NULL DEFAULT 'pending',
+    started_at    TIMESTAMP,
+    completed_at  TIMESTAMP,
+    summary_json  TEXT    NOT NULL DEFAULT '{}',
+    UNIQUE (run_id, stage_name, ordinal)
+);
+CREATE INDEX IF NOT EXISTS idx_sub_steps_run ON sub_steps(run_id, stage_name, ordinal);