feat: web automation wrapper (#1349), DATALINK table on web, System theme; honest decode accounting

Ported from 2c38de53 (#1717). Main already carries the hatch PDF draw
order fix (#1345), the numeric set_properties compare (#1348) and the
web open from bytes, so those parts stay out; what remains:

#1349 — window.ocsControl.ready/request promise wrapper with an
ocs-ready event (web/ocs-control.js, loaded by the web page), and the
startup donation prompt is suppressed once a control request is seen
(opt-in pre-boot via localStorage["opencadstudio.automation"] = "1").
docs/automation/web.md documents both. The control protocol stays at 1.

DATALINK CSV-to-table works on web (picker + bytes through the session
registry — was std::fs, silently dead); COLORSCHEME accepts System
(prefers-color-scheme on web, AppsUseLightTheme on Windows).

Decode accounting, made honest per the fix-of-fix review: into_rgba8
allocates the RGBA destination while the native buffer is alive for
every non-Rgba8 layout (L8 5, La8 6, Rgb8 7, L16 6, La16 8 B/px
peaks — the formula now says so), GIF bills the screen+frame
coexistence at 8 B/px, and TIFF drops the 3x multipliers both ways —
tiff 0.11 splits the given budget between frame and scratch (a total
ceiling), so the old double-billing refused honest 16-bit GeoTIFFs
~2.25x early while admitting hostile chunk transients at 3x.

(cherry picked from commit 2c38de53)

src/io/mod.rs

@@ -273,6 +273,27 @@ pub async fn pick_linetype_file_web() -> Option<(String, String)> {
     Some((name, String::from_utf8_lossy(&bytes).into_owned()))
 }
 
+/// Web DATALINK: pick the .csv through the browser and read its raw bytes —
+/// the stand-in for the typed path the native command reads with
+/// `std::fs::read_to_string`. The caller registers the bytes in the
+/// picked-source registry so the link re-reads in-session. `None` = the user
+/// cancelled.
+#[cfg(target_arch = "wasm32")]
+pub async fn pick_datalink_csv_web() -> Option<(String, Vec<u8>)> {
+    let Some(handle) = crate::sys::file_dialog()
+        .set_title(crate::t!("Link a CSV data source").as_ref())
+        .add_filter(crate::t!("CSV Files").as_ref(), &["csv", "txt", "CSV", "TXT"])
+        .add_filter(crate::t!("All Files").as_ref(), &["*"])
+        .pick_file()
+        .await
+    else {
+        return None;
+    };
+    let name = handle.file_name();
+    let bytes = handle.read().await;
+    Some((name, bytes))
+}
+
 /// Load a CAD file from a known path. Parsing and cache building run on a
 /// dedicated OS thread so the async executor stays free for rendering during
 /// the load. Writes phase markers into `phase` so the UI can show