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)

web/ocs-control.js

@@ -0,0 +1,72 @@
+// Web automation convenience API (#1349).
+//
+// The wasm module exposes the control bridge through `window.wasmBindings`
+// (ocs_control_submit / ocs_control_take). This module wraps it in promises:
+//
+//   await window.ocsControl.ready            resolves once the bridge exists
+//   window.addEventListener("ocs-ready", …)  fired once, at the same moment
+//   await window.ocsControl.request({...})   one control request -> final reply
+//
+// Hosts that intend to automate from the start should set
+// localStorage["opencadstudio.automation"] = "1" before the page loads so the
+// startup donation prompt never appears (see docs/automation/web.md).
+(() => {
+  if (window.ocsControl) return;
+  const bridge = () => {
+    const api = window.wasmBindings;
+    return api && api.ocs_control_submit && api.ocs_control_take ? api : null;
+  };
+  const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));
+
+  let readyResolve;
+  const ready = new Promise(resolve => { readyResolve = resolve; });
+  const settle = () => {
+    readyResolve(window.ocsControl);
+    window.dispatchEvent(new Event("ocs-ready"));
+  };
+  const poll = setInterval(() => {
+    if (bridge()) {
+      clearInterval(poll);
+      settle();
+    }
+  }, 50);
+  if (bridge()) {
+    clearInterval(poll);
+    settle();
+  }
+
+  // Submit one request and poll take() until the app stores its reply.
+  async function exchange(api, payload, pollMs, deadline) {
+    const ticket = api.ocs_control_submit(JSON.stringify(payload));
+    for (;;) {
+      const raw = api.ocs_control_take(ticket);
+      if (raw) return JSON.parse(raw);
+      if (performance.now() >= deadline) {
+        throw new Error("ocs-control: timed out: " + JSON.stringify(payload));
+      }
+      await sleep(pollMs);
+    }
+  }
+
+  // One control request. Replies that are still `accepted`/`running` (an
+  // async open, capture or save) are followed through `op:"operation"` until
+  // a final status arrives. Resolves with the final reply object; rejects
+  // only on timeout or a missing bridge.
+  async function request(payload, options = {}) {
+    const pollMs = options.pollMs ?? 50;
+    const deadline = performance.now() + (options.timeoutMs ?? 120000);
+    const api = bridge();
+    if (!api) {
+      throw new Error("ocs-control: bridge not ready (await ocsControl.ready)");
+    }
+    let reply = await exchange(api, payload, pollMs, deadline);
+    while (["accepted", "running"].includes(reply.status) && reply.request_id) {
+      await sleep(pollMs);
+      reply = await exchange(
+        api, {op: "operation", request_id: reply.request_id}, pollMs, deadline);
+    }
+    return reply;
+  }
+
+  window.ocsControl = {ready, request};
+})();