Add mobile-viewport testing to run-server + require it in CLAUDE.md

Confirmed the theatre-mode preview dialog actually renders correctly
at phone widths (390x844) -- centers properly, backdrop and close
button both fine, nothing overflows. Added a `viewport` command to the
driver (defaults to mobile, since that's the step easy to skip) and a
CLAUDE.md rule to screenshot both breakpoints for future UI changes,
since the 860px sidebar/mobile-bar fork is a real, previously-hit
source of bugs here.

CLAUDE.md

@@ -54,7 +54,16 @@ Start here, don't re-derive from scratch:
   live (mobile viewport CSS collapse, a dialog's status message landing
   behind its own backdrop, a JSON/form-urlencoded body mismatch). Spin up
   `uvicorn app.main:app` against a scratch `DATABASE_URL`/`CONFIG_PATH`
-  sqlite file, don't touch the real deployment's data.
+  sqlite file, don't touch the real deployment's data. `server/.claude/skills/run-server/`
+  (`/run-server`) has a driver for exactly this.
+- **New/changed UI must work at both desktop and mobile widths --
+  screenshot both, don't assume one implies the other.** The layout
+  genuinely forks at the 860px breakpoint (`theme.css`): the sidebar
+  goes off-canvas behind a hamburger below it. A dialog, header
+  control, or new widget that looks right at a wide viewport can
+  overflow, overlap the mobile bar, or mis-center at phone widths.
+  `run-server`'s driver has a `viewport` command for exactly this
+  (defaults to a phone size; switch to `1280 900` for desktop).
 - **Deploy**: Gitea Actions at `git.thumeit.com/tfaour/espresso_frame`
   (`.gitea/workflows/server-docker-build.yml`: `test` -> `build-and-push`
   -> `deploy` on any push to `main` touching `server/**`; `deploy` SSHes