Export: Add advanced client-side print mode

[dopenguin]

Description

The export plugin currently only lets users click a single format, which then downloads/opens in a new tab or is stored in the plugin store. The output is a simple screenshot at screen resolution in a fixed A4 landscape layout which is not suitable for proper printouts.

This feature adds an opt-in, fully-featured, purely client-side print mode on top of it without any dependency on a MapFish print server.

A new configuration parameter advanced: boolean (default false) controls the behavior:

• advanced: false: behaves exactly as today. Click a format, result is downloaded/opened or stored.
• advanced: true: shows a print interface where users can configure the export before generating it.

In advanced mode, users should be able to choose the following options:

• Format (e.g. DIN A5–A0, portrait/landscape)
• DPI/resolution of the map image; low (96 dpi), medium (150 dpi) or high (300 dpi)
• optional: title (free text input); not set if empty
• optional: tooter (free text input); not set if empty
• optional: legend of the active layers
• optional: source attribution (attributions); only possible if attributions is configured

The result is generated, saved in the store and downloaded in the selected format with the map and selected components being arranged in a print-ready layout.

Affected areas

File	What needs to change
src/plugins/export/types.ts	Currently only knows download and formats; add advanced flag plus new options described above.
src/plugins/export/store.ts	Extend exportAs with state for above options; add print rendering for advanced mode; keep the existing screenshot path for advanced: false
src/plugins/export/utils/convertToPdf.ts	Remove hard-wired A4 landscape (const format = 'a4'); support format (default 'a4'/orientation/DPI; place title, footer, legend, source attribution into the PDF layout
src/plugins/export/utils/getCanvasFromMap.ts	Enable rendering at target resolution (DPI) instead of the current viewport/screen resolution
src/plugins/export/components/ExportUI.ce.vue	Render the options form when advanced: true, keep the plain format buttons when advanced: false; add separate components for both modes
src/plugins/export/locales.ts	New labels/tooltips for all new options
src/plugins/layerChooser/utils/findInCapabilities.ts	Abstract the function to obtain legend URLs of the active layers for printing

Out of scope

• No connection to a MapFish print server or other server-side print backends.
• No changes to the default (advanced: false) behavior beyond keeping it intact.
• No freely definable, fully customizable print templates (only the components listed above).
• No export of feature/attribute tables beyond the legend.