docs: web platform + clean URLs + dark/light mode (#37)

* chore(docs): replace d2 diagram pipeline with mermaid

Remove docs/_diagrams/ and d2 build step from Makefile. The HTML
template already initialises mermaid.js; diagrams are now inline
code fences rendered client-side.

* docs(architecture): add mermaid diagram + web/CDP platform docs

Replace SVG img tag with inline mermaid flowchart showing both native
(Maestro sidecar + in-app SDK) and web (Chrome CDP) paths. Update
Processes, Transports table, and per-step cycle sections.

* docs(design-principles): update principles 1-4 for web platform

Principles 1, 2, 3, and 4 referenced Maestro and native-only concepts.
Add web/CDP context and update driver-is-an-interface to name both
sidecar and chrome implementations.

* docs(manual): add web prerequisites and folio-web example

Update --platform flag to list android, ios, web. Add web prerequisites
section (Chrome, no SDK needed) and folio-web quick-start to
getting-started.

* chore(gitignore): untrack inspect dist/index.html build artifact

index.html is regenerated by vite on every build with a new content hash,
making it permanently dirty. Only .gitkeep is needed for //go:embed to
compile on a fresh checkout. Also remove duplicate dist/* line and stale
d2 diagram ignore entries.

* feat(docs): click-to-zoom for mermaid diagrams

* docs(architecture): change diagram layout from LR to TB

* docs(getting-started): link npm and Maven Central package headers

* update docs root

* docs(spec): rewrite npm package README

Update usage example to current API, drop stale version-compatibility
and license sections.

* build(docs): output pages as pagename/index.html for clean URLs

Split DOCS_OUT into INDEX_OUT (index.md files stay as index.html) and
PAGE_OUT (all other pages become pagename/index.html). The __ROOT__
depth computation already handles the extra directory level correctly.

* chore(docs): update sidebar links to directory-style URLs

* docs: update cross-links from .html to directory-style paths

* ci(docs): remove d2 install step

* feat(docs): dark/light mode toggle

Add theme toggle button (top-right, fixed). Persists preference in
localStorage; falls back to prefers-color-scheme. Flash-free via inline
script in <head> that sets data-theme before first paint.

* fix(docs): fix inspect image path broken by directory URL restructure

* feat(docs): click-to-fullscreen for all article images

* fix(inspect): allow AssetsFS override in ServerOptions; drop unused request param from serveIndex

* fix(inspect): use in-memory FS in tests so TestAssets_FallbackToIndexHTML passes without web build

docs/_assets/style.css

@@ -21,6 +21,24 @@
   }
 }
 
+html[data-theme="light"] {
+  --bg: #ffffff;
+  --fg: #2a2a2a;
+  --muted: #6a6a6a;
+  --border: #e5e5e5;
+  --accent: #0a66c2;
+  --code-bg: #f6f6f6;
+}
+
+html[data-theme="dark"] {
+  --bg: #0f0f10;
+  --fg: #e5e5e5;
+  --muted: #a0a0a0;
+  --border: #2a2a2a;
+  --accent: #4aa3ff;
+  --code-bg: #1a1a1b;
+}
+
 * { box-sizing: border-box; }
 
 html { background: var(--bg); color: var(--fg); }
@@ -114,6 +132,7 @@ main article img {
   display: block;
   margin: 1.25rem 0;
   border-radius: 4px;
+  cursor: zoom-in;
 }
 
 a { color: var(--accent); }
@@ -184,8 +203,66 @@ footer a { color: var(--muted); }
 .mermaid {
   text-align: center;
   margin: 1.5rem 0;
+  cursor: zoom-in;
 }
 
+.diagram-overlay {
+  display: none;
+  position: fixed;
+  inset: 0;
+  background: rgba(0, 0, 0, 0.9);
+  z-index: 9999;
+  cursor: zoom-out;
+  align-items: center;
+  justify-content: center;
+  padding: 2rem;
+}
+
+.diagram-overlay.open {
+  display: flex;
+}
+
+.diagram-overlay svg {
+  max-width: 95vw;
+  max-height: 95vh;
+  width: auto;
+  height: auto;
+  background: var(--bg);
+  border-radius: 6px;
+  padding: 1.5rem;
+}
+
+.theme-toggle {
+  position: fixed;
+  top: 1rem;
+  right: 1rem;
+  z-index: 100;
+  background: var(--code-bg);
+  border: 1px solid var(--border);
+  color: var(--fg);
+  cursor: pointer;
+  padding: 0.35rem;
+  border-radius: 6px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 2rem;
+  height: 2rem;
+  transition: border-color 0.15s;
+}
+
+.theme-toggle:hover { border-color: var(--muted); }
+
+.theme-toggle svg {
+  width: 1rem;
+  height: 1rem;
+}
+
+html[data-theme="light"] .icon-sun  { display: none; }
+html[data-theme="light"] .icon-moon { display: block; }
+html[data-theme="dark"]  .icon-sun  { display: block; }
+html[data-theme="dark"]  .icon-moon { display: none; }
+
 @media (max-width: 768px) {
   .layout { grid-template-columns: 1fr; }
   .sidebar {