Update Highstock/Highcharts to 12.6.0

[Salvialf]

• Updates the vendored Highstock/Highcharts library from 10.3.2 to 12.6.0 (not 13.0.0, due to a foreignObject/useHTML rendering incompatibility discovered in v13, plus a number of other open issues still unresolved in that version).
• Cleans up the vendored library footprint down to only the files actually used (modules, lib, core builds), going from 768 files / 13.1 MB to 85 files / 3.0 MB.
• Updates the lib/ export dependencies bundled with Highstock: jspdf.js (2.5.0 → 4.2.1), plus canvg.js and svg2pdf.js; rgbcolor.js is no longer needed and has been removed.
• Highcharts.Color(...) now requires new Highcharts.Color(...), updated at all 3 call sites.
• Fixes an axis padding regression that could show a tick/gridline for a date past the last real data point (e.g. tomorrow's date with no data), by removing the minPadding/maxPadding overrides on the main x/y axes, matching Highstock's actual default of 0.
• Fixes a scrollbar rendering bug (negative-size rectangle) caused by the scrollbar always being enabled internally regardless of the showScrollbar option; it now follows showScrollbar like before.
• Restores full-length day/week/month names in axis labels and tooltips: 12.6.0 switched several date format defaults to a new bracket-syntax (%[eb]) that resolves via Intl and ignores our translated lang.shortMonths, so these are now pinned back to the classic token syntax (%e %B, %B '%y).
• Removes the now-dead lang.months/shortMonths/weekdays translation arrays: since lang.locale is set, 12.6.0 generates correct, grammatically accurate values from Intl automatically (including proper French lowercase month names), making the old hardcoded translations redundant.
• Adds French translations for the range selector's new default tooltips (allTitle/monthTitle/yearTitle), introduced in 12.2.0, which were previously showing in English ("View all", etc.).
• Restores the pre-upgrade axis label size/spacing (fontSize, tickLength, label distance) after several small layout defaults changed between versions.
• Updates the two custom y-axis toggle button icons (divers-viral → fa-object-group, fa-ruler-vertical → fa-border-style) and adds a CSS override for icon centering inside the small chart toolbar buttons, both affected by internal font/label positioning changes in the new renderer.
• Enables native chart export (PNG/JPEG/PDF/SVG/CSV/XLS) by turning on the offline-exporting and export-data modules, previously disabled, and disables the exported data table's caption row.
• Fixes the CSP to allow blob: image sources, required for native PNG/JPEG export to work, and lets native download links bypass the SPA navigation interceptor so exported files actually download instead of being caught by client-side routing.
• Removes a dead CSS rule (.highcharts-tooltip .highcharts-shadow) that targeted a class no longer produced by the new renderer.
• Existing installs pick up the updated CSP automatically on their next core update via the existing install/consistency.php self-healing check, no new update script needed.