feat: responsive mobile layout for monitoring dashboard

Redesign the UI to work on mobile/portrait viewports using CSS media
queries at max-width 768px:

- Host rows stack vertically: info (name, latency, status) on top,
  sparkline chart full-width below
- Summary stats line wraps properly with hidden pipe separators
- Header stacks title and controls vertically
- Pause button and controls sized appropriately for touch
- Pin button repositioned for mobile touch targets
- Footer legend wraps cleanly

Desktop layout remains pixel-identical — all changes are scoped to the
@media (max-width: 768px) query in styles.css only.

Refs #2

TODO.md

@@ -1,47 +0,0 @@
-# Workflow
-
-* branch (from `main`)
-* do the work in Next Step
-* move Next Step to the top of Completed Steps
-* move the top item of Future Steps into Next Step
-* commit (`TODO.md` changes in the same commit as the work)
-* merge to `main` if the branch is not protected, otherwise open a PR
-* push
-
-# Status
-
-pre-1.0. No git tags. Backend work in flight on feat/reportbuf-storage
-(dirty: src/main.js). Frontend is functional; backend is new and unmerged.
-
-# Next Step
-
-Land feat/reportbuf-storage: finish the in-progress src/main.js change,
-get make check green, and merge the branch to main. The branch adds the
-backend (buffered zstd-compressed report storage), the CI workflow, and
-backend repo standard files, so merging it also closes most compliance
-gaps.
-
-# Completed Steps
-
-- 2026-02-27: backend with buffered zstd-compressed report storage; CI
-  workflow and backend repo standard files; backend Dockerfile fixed (Go
-  1.25, golangci-lint) and moved to repo root (feat/reportbuf-storage,
-  unmerged)
-- 2026-02-26: host row layout redesigned with CSS grid; overflow and
-  spacing fixes; nginx config extracted; port hardcoded to 8080
-- 2026-02-26: debug log panel, median stats, recovery probe, Docker
-  build fix, S3 Singapore endpoint added
-- 2026-02-23: summary box redesign, host pinning, local and UTC clocks,
-  checks counter
-- 2026-02-23: hosts sorted by latency; GET instead of HEAD for latency;
-  timeout derived from interval; Hetzner regional endpoints; 3s interval
-- 2026-01-29: initial NetWatch network latency monitor
-
-# Future Steps
-
-- Compliance top-up as one small commit: add .editorconfig and add the
-  hooks target to the Makefile
-- After merge, confirm .gitea/workflows/check.yml is on main and CI is
-  green (main always green policy)
-- Decide what to do with untracked resume.sh: commit it, gitignore it,
-  or delete it