chore(dashboard): fix bugs found in dashboard review

[TimVosch]

Fixes a batch of bugs found during a review of the dashboard web app. Each fix was developed in an isolated worktree and merged together; the combined branch builds, vets, and is gofmt-clean.

Fixes

Critical

• Pipeline "Create" page crash — the traces panel dereferenced p.Pipeline.Id unconditionally; on /pipelines/create Pipeline is nil, panicking mid-render. The panel is now guarded with {% if p.Pipeline != nil %}. (views/pipelineEditPage.qtpl)

High

• Device list pagination dropped the sensor-group filter — createSensorGroup/deleteSensorGroup passed a bare cursor where a URL was expected (breaking infinite scroll), and getDevicesTable/deviceListPage omitted sensor_group from the next-page URL (page 2+ returned all devices). All four spots now build full …?sensor_group=&cursor= URLs (delete correctly omits the filter). (routes/overview.go)
• Datastream websocket missing returns — bad start/end params wrote an HTTP error then still tried to upgrade the socket on a committed response. Added return after each parse error. (routes/overview.go)

Medium

• updatePipeline error handling — read resp.Body when resp could be nil, fell through to a nil-deref status check, and had an unreachable errors.As branch. Rewritten to be nil-safe and return after surfacing the error. (routes/pipelines.go)
• Trace timeline panic — trace.WorkerTimes[j] could index out of range when the API returns fewer times than workers. Added a bounds guard. (routes/ingress.go)
• Worker description couldn't be cleared — empty form values were skipped; description is now always sent. (routes/workers.go)
• Create-worker ignored the Enabled toggle — createWorker now reads state and sets it. (routes/workers.go)

Low / robustness

• Added middleware.Recoverer so handler/template panics return a clean 500 instead of dropping the connection. (main.go)
• Fixed formatSince copy ("About 1 hour ago" / "About 1 minute ago") and removed a duplicated branch. (routes/ingress.go)
• Closed websocket frame writers per-iteration instead of defer-ing in a loop. (routes/overview.go)
• Removed stray fmt.Print* debug statements. (workers.go, pipelines.go, main.go)

Verification

go build ./services/dashboard/..., go vet ./services/dashboard/..., and gofmt -l all clean.

Not included (cosmetic / larger calls)

Worker name field styled to look disabled but editable; empty 200 on non-HTMX table endpoints; external CDN dependencies; unescaped server data in inline <script> strings; dead Trace.PipelineName.

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