fix: joined button-group corners and dark-mode contrast (v2.1.2)

[ehsanjavaid]

Fixes #22.

What was wrong

• Grouped list-view buttons (Filter/clear-filter, sort field/sort-order) rendered as visually separate pills instead of one joined control — the theme's uniform border-radius on .btn-primary/.btn-default/.btn-secondary was overriding Bootstrap's corner-flattening for .btn-group children.
• In dark mode, the active "Filters" button label was unreadable (near-black text on a near-black background) — --bg-dark-gray had collapsed to the page background instead of staying a lighter chip surface.
• In dark mode, unchecked checkboxes rendered as the browser's default white box — Frappe's checkbox CSS never sets an unchecked background color.

Fix

• Restore Bootstrap's .btn-group corner-flattening, re-asserted after the theme's blanket button radius.
• Map --bg-dark-gray to the same lighter --gray-500 token stock Frappe's dark theme uses for that pairing.
• Give unchecked checkboxes an explicit dark background in dark mode.
• Bump app_version to 2.1.2 and the solvronix_desk.css/dark_mode.css cache-busters.

Verified against a live browser session (screenshots + computed-style checks) on both erp.solvronix.com and poultry-staging.solvronix.com. All 116 Python + 180 JS tests pass.