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.

server/.claude/skills/run-server/SKILL.md

@@ -89,6 +89,20 @@ Screenshots land in `/tmp/run-server-shots/` (override:
 `SCREENSHOT_DIR`). **Actually look at them** -- a blank or error-page
 screenshot is a failure to launch, not success.
 
+**Test every UI change at both a desktop and a mobile viewport.** The
+driver defaults to a desktop size (1280x900); switch with `viewport`.
+The app's mobile breakpoint is 860px (`theme.css`) -- below that the
+sidebar goes off-canvas behind a hamburger (`.mobile-bar`). A page that
+looks right at 1280px can still overflow, overlap the mobile bar, or
+mis-center a `<dialog>` at phone widths -- screenshot both:
+
+```bash
+tmux send-keys -t runserver 'viewport 1280 900' Enter   # desktop (also the default)
+tmux send-keys -t runserver 'screenshot desktop-x' Enter
+tmux send-keys -t runserver 'viewport 390 844' Enter     # iPhone-ish mobile width
+tmux send-keys -t runserver 'screenshot mobile-x' Enter
+```
+
 ### Driver commands
 
 | command | what it does |
@@ -101,6 +115,7 @@ screenshot is a failure to launch, not success.
 | `screenshot [name]` | → `/tmp/run-server-shots/<name>.png` |
 | `eval <js>` | evaluate JS in the page, prints JSON |
 | `console-errors` | prints all captured console/page errors as a JSON array |
+| `viewport [w] [h]` | resize the viewport, default `390 844` -- use `1280 900` for desktop (see Gotchas re: real touch input) |
 | `is-open <dialog-selector>` | prints `true`/`false` for a `<dialog>` element's `.open` -- use this instead of `wait-for sel[open]` |
 | `bootstrap-admin [user] [pass]` | completes first-run `/setup` (defaults `admin`/`testpassword123`); links frame #1 and logs in |
 | `quit` | closes the browser, exits the driver |
@@ -138,6 +153,15 @@ needed beyond the venv.
 
 ## Gotchas
 
+- **`viewport` only resizes the window -- it does not emulate touch
+  input.** `click` still dispatches a mouse click, not a tap; there's
+  no touch-delay, no `:hover`-stickiness-after-tap, no `hasTouch`
+  context. It catches real bugs (layout overflow, off-canvas sidebar,
+  a `<dialog>` mis-centering at phone widths) but won't catch anything
+  that's specifically a touch-vs-mouse event difference. Good enough
+  for CSS/layout verification; not a substitute for testing on an
+  actual phone if a change touches touch-specific interaction.
+
 - **`chrome-headless-shell` (Playwright's default headless target)
   crashes on basic calls in this container**, e.g. `page.set_content()`
   returns `TargetClosedError`, even after every `ldd`-reported missing

server/.claude/skills/run-server/driver.py

@@ -95,6 +95,19 @@ def cmd_console_errors(_arg):
     print(json.dumps(console_errors))
 
 
+def cmd_viewport(arg):
+    """Resize the viewport. No args -> 390x844 (iPhone-ish mobile
+    width); the page itself starts at 1280x900 (desktop) on launch, so
+    `viewport 1280 900` gets back to that. The app's mobile breakpoint
+    is 860px (see theme.css) -- anything under that exercises the
+    off-canvas sidebar/mobile-bar layout."""
+    parts = arg.split()
+    width = int(parts[0]) if len(parts) > 0 else 390
+    height = int(parts[1]) if len(parts) > 1 else 844
+    page.set_viewport_size({"width": width, "height": height})
+    print("viewport:", width, "x", height)
+
+
 def cmd_is_open(arg):
     """App-specific: print whether a <dialog> element is open (true/false)."""
     print(json.dumps(page.eval_on_selector(arg, "el => el.open")))
@@ -132,6 +145,7 @@ COMMANDS = {
     "screenshot": cmd_screenshot,
     "eval": cmd_eval,
     "console-errors": cmd_console_errors,
+    "viewport": cmd_viewport,
     "is-open": cmd_is_open,
     "bootstrap-admin": cmd_bootstrap_admin,
     "quit": cmd_quit,