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/app/startup.rs

@@ -8,7 +8,13 @@ impl OpenCADStudio {
             self.pending_startup_modals
                 .push_back(ModalKind::AssocPrompt);
         }
-        if self.donation_prompt_version != env!("OCS_APP_VERSION") {
+        // Automation sessions (#1349): once control traffic has arrived (or a
+        // web host opted in at boot) while control is enabled, the donation
+        // prompt is suppressed — a modal would block every automated
+        // operation until a client closed it by hand.
+        if self.donation_prompt_version != env!("OCS_APP_VERSION")
+            && !(self.control.enabled && self.control.automation_seen)
+        {
             self.pending_startup_modals
                 .push_back(ModalKind::DonationPrompt);
         }
@@ -226,6 +232,39 @@ mod tests {
         assert!(restarted.active_modal.is_none());
     }
 
+    #[test]
+    fn automation_sessions_skip_the_donation_prompt() {
+        // First control contact drops a still-queued donation prompt and
+        // stops later queueing while control stays enabled (#1349).
+        let mut app = OpenCADStudio::new_for_test();
+        app.default_assoc_prompted = true;
+        app.queue_startup_prompts();
+        assert_eq!(app.active_modal, Some(ModalKind::DonationPrompt));
+        let _ = app.update(Message::CloseModal);
+        app.donation_prompt_version = "previous-release".to_string();
+        app.pending_startup_modals.push_back(ModalKind::DonationPrompt);
+        let _ = app.control_request(serde_json::json!({"op":"state"}));
+        assert!(app.pending_startup_modals.is_empty());
+        app.queue_startup_prompts();
+        assert_ne!(app.active_modal, Some(ModalKind::DonationPrompt));
+
+        // A still-queued (not yet shown) prompt is dropped on first contact.
+        let mut queued = OpenCADStudio::new_for_test();
+        queued.default_assoc_prompted = true;
+        queued.donation_prompt_version = "previous-release".to_string();
+        queued.pending_startup_modals.push_back(ModalKind::DonationPrompt);
+        let _ = queued.control_request(serde_json::json!({"op":"state"}));
+        queued.queue_startup_prompts();
+        assert_ne!(queued.active_modal, Some(ModalKind::DonationPrompt));
+
+        // A prompt already on screen is left for its reader; clients close
+        // it themselves, and a session with no control traffic still prompts.
+        let mut human = OpenCADStudio::new_for_test();
+        human.default_assoc_prompted = true;
+        human.queue_startup_prompts();
+        assert_eq!(human.active_modal, Some(ModalKind::DonationPrompt));
+    }
+
     #[test]
     fn startup_dialogs_wait_their_turn_and_preserve_unseen_prompts() {
         let mut app = OpenCADStudio::new_for_test();