Issue #3611851: Keep the selection panel visible, show the slot context, translate the picker and fix map panning

Frank Mablyrequested to merge
issue/yoyaku-3611851:3611851-keep-selection-panel-visible into 1.x

Follow-up to the graphical seat-map picker (#3611792).

  • Keep the "Your selection" panel visible at all times (a short hint when empty) so the map does not reflow on the first pick.
  • Show the slot context: the event name in the page title (H1 and browser title, for accessibility) and the date/time + venue as a subtitle below it; the same context also appears inside the map when it is full screen.
  • Make the picker fully translatable: the JS now calls Drupal.t() directly (a local t() alias hid the strings from Drupal's string extraction) and French translations are added to translations/yoyaku.fr.po.
  • Fix a panning bug where the map stuck to the cursor after the button was released outside the viewport (pointer capture plus a no-button-pressed guard).

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