Ignore AltGr keystrokes in Ctrl+Alt keyboard shortcuts (#825)

* Ignore AltGr keystrokes in Ctrl+Alt keyboard shortcuts

Browsers report AltGr (right Alt on AZERTY/QWERTZ and most non-US
layouts, used to type @ # { } [ ] | \ and the euro sign) as
ctrlKey+altKey. The default keybinds map destructive actions to
Ctrl+Alt+<letter> (delete_session, new_session, incognito,
open_calendar), so a non-US user typing a special character could
silently fire them.

Guard the shortcut matcher, the editor keydown handler, and the rebind
capture with getModifierState('AltGraph'), which is true for AltGr but
false for a genuine left Ctrl+Alt. macOS is excluded: there the Option
key legitimately sets AltGraph and there is no AltGr/Ctrl+Alt collision
to guard against, so the guard would otherwise break Ctrl+Option /
Cmd+Option shortcuts (notably in Firefox).

The detection lives in one place — isAltGrEvent / IS_MAC in
static/js/platform.js — and all three call sites route through it, so the
guards can't drift apart.

The editor handler only skips the Ctrl+Alt chord block, so layout
shortcuts reachable via AltGr (e.g. [ ] brush size = AltGr+5/+8 on
AZERTY) keep working.

* Require Ctrl+Alt for the AltGr guard and consolidate keybind test marks

isAltGrEvent now also checks ctrlKey+altKey so it only suppresses the
"AltGr reported as Ctrl+Alt" collision; an event asserting AltGraph on
its own (a Linux ISO_Level3_Shift layout, a stray modifier) is left
alone. Pin it with test_isaltgr_false_when_altgraph_set_but_not_ctrl_alt.

Collapse the 12 per-test node skipif marks into one module-level
pytestmark, and note in platform.js why IS_MAC intentionally covers
iPad/iPhone and mirrors the isMac checks in calendar.js / sessions.js.

static/js/editor/keyboard-shortcuts.js

@@ -50,6 +50,7 @@
  * }} deps
  */
 import { state } from './state.js';
+import { isAltGrEvent } from '../platform.js';
 
 export function wireKeyboardShortcuts(deps) {
   const {
@@ -79,7 +80,11 @@ export function wireKeyboardShortcuts(deps) {
       return;
     }
     if (e.key === 'Escape') return;
-    if (e.ctrlKey || e.metaKey) {
+    // Skip the Ctrl+Alt editor chords for an AltGr keystroke (see platform.js);
+    // only the chord block is skipped, so the layout-character handlers below
+    // still act — AltGr+5 / AltGr+8 stay as the [ ] brush-size shortcut on
+    // AZERTY / QWERTZ.
+    if ((e.ctrlKey || e.metaKey) && !isAltGrEvent(e)) {
       if (e.key === 'z') { e.preventDefault(); if (e.shiftKey) redo(); else undo(); }
       // Ctrl+Shift+D = Deselect: clears the wand selection (and
       // lasso if active) without affecting layers.