feat(dashboard): replace the build step table with one log stream - #7689

tldr: build logs are one list now, Vercel style, read through
getBuildLogs. The step table, its tRPC route and its ClickHouse
query are gone.

  > [builder 7/10] RUN go build    <- header when stepId changes
  12:00:01.120  added 312 packages
  12:00:06.410  ✓ DONE 5.3s
  > [builder 8/10] RUN npm test
  12:00:06.500  npm warn ...       <- stderr, amber
  12:00:07.010  ERROR: exit 1      <- red, Jump to error target

- Lazy: pages load about 300 lines ahead of the viewport.
- A finished run of more than 40 entries folds to its first and last
  5 behind "Show N more entries". Runs with an error and the last run
  stay open.
- Jump to latest and Jump to error read every remaining page in one
  fetch behind a button spinner, then scroll once. A failed build and
  a build that is still running read to the end the same way. Pages
  ask for 500 entries.
- A long read publishes what it has every 250ms, so slow pages show
  progress without a render per page.
- A running build follows the tail. Only an upward scroll stops it,
  because rows grow below the viewport when they are measured. A
  finished build opens at the top.
- Polling backs off on a quiet build: 1s, then 2s after an empty poll.
- Loading keeps the final frame (toolbar and list height) with a
  spinner, so nothing jumps. "Loading more" sits at the end while
  more pages exist.
- The build card opens on its own once the steps show a fresh build.
- Rows show the output as a terminal would: ANSI codes are stripped,
  a carriage return redraws its line, and a chunk that continues an
  unfinished line of the same step and stream joins it. ctrl's
  CACHED / DONE / ERROR lines never join. The API keeps the raw text.
- The list uses @tanstack/react-virtual directly. VirtualTable has
  fixed row heights and no scroll to index. Rows render a plain
  <time>, not TimestampInfo.

The cursor only moves forward, so reaching the end still reads every
page. That is about 4 requests for a 2000 entry build.

- build-steps-table/, use-build-steps.ts
- tRPC deploy.deployment.buildSteps
- @unkey/clickhouse buildSteps.getSteps (build-steps.ts)
- the failed build screenshot in build-logs.mdx, it showed the table
- StreamingTable onRowClick and renderExpanded, only the table used them
- the error-blink animation in theme.css, only the table used it

While a build runs, the build status says "Building..." instead of
the current step name, which came from the step table query.

The dashboard now needs deployments.getBuildLogs, so the API from
PR 3 ships first.