fix(sidebar): bound keyboard backlight scale marks

[perlowja]

Summary

• The keyboard-backlight brightness slider in SystemView created one GtkScale mark per raw hardware brightness level. On boards with a high-resolution backlight controller (Apple T2 reports max_brightness=14660), that's 14,661 marks, which made GTK compute a ~14,662px minimum widget width.
• That propagated to a ~14,794px sidebar width, and at 2x HiDPI scale the compositor attempted a 29588x1406px EGL surface -- past Mesa's max texture size. The allocation failed, leaving Mesa's Wayland EGL back-buffer image null, and dri2_query_image_common() crashed dereferencing it.
• Scope: bound the number of decorative tick marks to a small constant (10), keeping all 14,660 real brightness increments via set_increments() -- only the number of drawn marks changes, not slider precision or behavior.
• Blast radius: system_view.vala's keyboard-backlight slider only; no other sidebar widgets touched.

Validation Evidence

Root-caused from real coredumps on a MacBookPro16,1 (T2, AMD Navi14) running this shell:

Program terminated with signal SIGSEGV, Segmentation fault.
#0  dri2_query_image_common (image=0x0, attrib=8196 [__DRI_IMAGE_ATTRIB_WIDTH], ...)
    at ../src/gallium/frontends/dri/dri2.c:1046

Six independent coredumps all showed the identical 29588x1406 EGL buffer request. Confirmed the keyboard-backlight slider as the source by reading this file's mark-generation loop against the real hardware's reported max_brightness=14660.

Built and installed the patched binary on the same real hardware:

• Ran across 2 full compositor/login restarts.
• Opened the sidebar (activating the keyboard-backlight slider) 12 times per session.
• Zero new coredumps in either session; the shell process stayed alive throughout.

This is a genuine upstream Mesa robustness gap too (missing NULL check after a failed EGL Wayland back-buffer allocation) -- already tracked as Mesa issue #15913 with an open, unmerged fix (MR !44569). This PR avoids generating the invalid surface in the first place, which is the fix within this repo's control; the Mesa MR remains valuable defense-in-depth regardless.

Security & Privacy

No security or privacy impact -- pure UI layout bound, no new data handling.

Compatibility

No behavior change for normal (low-resolution) brightness controllers. Preserves exact slider precision/increments for high-resolution controllers; only the number of visual tick marks is reduced.

Rollback

Revert this commit; no migrations or state changes involved.

AI assistance: disclosed

[perlowja]

@mirkobrombin bumping this one too - it's a real crash fix (keyboard-backlight GtkScale allocating a 29588x1406 EGL surface on high-resolution backlight controllers), metal-confirmed fixed.