docs(qa): web UI manual QA pass + fix grid column overflow (closes #28)

Documents and verifies the web UI against all 10 acceptance criteria
from issue #28, plus one visual bug surfaced during the pass and
fixed in the same PR.

Bug fix:
* `.main-content` grid column was overflowing the viewport on the
  right whenever a bubble contained an unbreakable child (long code
  block, long URL, long tool-call output). Header + title rendered
  fine because they live outside the grid. Cause: grid items default
  to `min-width: auto`, which refuses to shrink below intrinsic
  min-content. Fix: add `.main-content { min-width: 0 }` so the
  existing `overflow-x: auto` on `.prose pre` and
  `word-break: break-all` on `.tool-call-content` can take over inside
  the bubble.

QA artifacts:
* `tests/web-ui-qa-checklist.md` — 9 sections covering: environment,
  server launch, 5 major views (home, workspace, conversation, search,
  config), exports, cross-browser parity, regression notes, sign-off.
  Every check that could be answered backend-side is filled in with
  the actual probe / inspection result.

* `tests/web-ui-smoke.sh` — boots app.py on an alt port and probes
  11 routes (page routes + JSON API + workspace-scoped routes).
  Exits non-zero on any unexpected status.

* `samples/qa/*.png` — 10 screenshots, Chrome + Firefox for home,
  workspace detail, conversation view, search results, and config.

Verification highlights:
  - 11/11 smoke probes pass
  - All 5 export formats download from both Chrome and Firefox
  - .md / .html / .json / .csv are byte-identical across browsers
    (proves the JS pipeline in static/js/download.js is portable)
  - PDF is a valid 153-page document (Chrome + Firefox both produce
    valid 153-page PDFs; bytes differ because the server embeds a
    fresh /CreationDate on each /api/generate-pdf call)
  - Backend data shape verified: a sampled conversation tab carries
    387 bubbles, including toolCalls (name="glob_file_search",
    status="completed") and thinking blocks (durationMs=3569)

Author: timon0305