fix(composer): recognisable path containment, and define md:inline

Follow-up to the previous commit, which made the CodeQL count worse
rather than better: 19 alerts became 22. Two mistakes.

First, the containment check used `base not in candidate.parents`.
That is correct Python but not a form static analysis recognises, so
every path-injection alert stayed and _plugin_dir itself picked up two
more. It now uses os.path.realpath plus os.path.commonpath, which is
both the documented sanitiser shape and stricter than the obvious
alternative: "/x/plugins-evil" startswith "/x/plugins" but is a
different directory, and there is now a test that fails if anyone
swaps commonpath for startswith.

Second, raising ComposerInputError from _plugin_dir and returning
str(exc) added two new py/stack-trace-exposure alerts -- CodeQL flags
exception text reaching a response regardless of the exception's type.
_plugin_dir returns None instead and the three handlers answer with a
fixed literal. There is nothing a caller needs there beyond "that id is
not ok".

Also defines .md\:inline in app.css. composer.html marks five toolbar
button labels `hidden md:inline`, and the class was never defined, so
those labels were hidden at every width and the buttons stayed
icon-only. main's test_web_static_audit.py catches it -- the branch
predates that test, which is why it only surfaced now that CI checks
the merge:

    Responsive utility classes referenced in templates but never
    defined in app.css (they silently no-op): ['md:inline']

Verified against the merged state -- main's app.css plus this one line,
audited against this branch's templates: 3 passed. The other twelve
classes the audit flags locally are defined on main and are artifacts of
this branch being 54 commits behind.

33 containment tests. Mutation-checked twice: removing the containment
lets eight payloads escape, including /etc/passwd and
plugin/../../../../../../etc/shadow; swapping commonpath for startswith
fails the sibling-prefix test.

Not addressed: three py/stack-trace-exposure alerts on the
_generate_plugin_files handlers. Those return str(exc) for
ComposerInputError, whose seven raise sites are all authored literals
("Author is required.", "Config variable key X is not a valid Python
identifier."). Suppressing them means replacing useful validation
feedback with a generic string, which is a real cost to the user for a
scanner's benefit. Worth a decision rather than a silent downgrade.

The 5 test_web_api.py failures are pre-existing on this branch --
identical counts with these changes stashed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01STMbQE4YctTacQXfbYqKuW

web_interface/static/v3/app.css

@@ -421,6 +421,9 @@ a, button, input, select, textarea {
     .md\:grid-cols-4 { grid-template-columns: repeat(4, minmax(0, 1fr)); }
     .md\:flex { display: flex; }
     .md\:hidden { display: none; }
+    /* composer.html labels its toolbar buttons `hidden md:inline`, so without
+       this the label is hidden at every width and the buttons stay icon-only. */
+    .md\:inline { display: inline; }
 }
 
 @media (min-width: 1024px) {