Tag Vivaldi's window class case-insensitively in browser.lua

[reverb256]

Fixes #9274.

Problem

Hyprland reports Vivaldi's window class in lowercase as vivaldi-stable, but the chromium-based-browser rule in default/hypr/apps/browser.lua spelled it with a capital V:

o.window("((google-)?[cC]hrom(e|ium)|[bB]rave-browser|[mM]icrosoft-edge|Vivaldi-stable|helium)", { tag = "+chromium-based-browser" })

Because the other alternatives already cover both cases ([cC]hrom, [bB]rave, [mM]icrosoft), Vivaldi never matches the rule, keeps the generic -default-opacity tag, and misses the tile = true + opacity = "1.0 0.985" treatment the other Chromium browsers get.

Change

Match the class in both cases, consistent with its neighbouring alternatives:

o.window("((google-)?[cC]hrom(e|ium)|[bB]rave-browser|[mM]icrosoft-edge|[vV]ivaldi-stable|helium)", { tag = "+chromium-based-browser" })

Test

Adds test/shell.d/vivaldi-window-class-test.sh, a regression guard that asserts browser.lua both tags a chromium-based-browser window class and matches Vivaldi case-insensitively (and fails if the case-sensitive spelling returns):

ok - browser.lua tags a chromium-based-browser window class
ok - Vivaldi's class is matched in both cases

• bash -n test/shell.d/vivaldi-window-class-test.sh — clean
• bash test/shell.d/vivaldi-window-class-test.sh — passes

Note: the test filename is scoped to Vivaldi to avoid colliding with #8968, which independently adds test/shell.d/browser-tags-test.sh for the Brave-origin rule.