Move plugin version into .env, fix version-bump workflow to only touch it

The old workflow grepped CMakeLists.txt for the first "VERSION X.Y" match,
which was cmake_minimum_required's own version rather than the project's,
mangling it to 3.27.0.0 on the first real bump. PLUGIN_VERSION in .env is
now the single source of truth for both CMakeLists.txt and flake.nix, and
the workflow's bump/commit steps only ever touch that one file.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CMakeLists.txt

@@ -1,8 +1,19 @@
 cmake_minimum_required(VERSION 3.27)
 
+# PLUGIN_VERSION lives in .env (single source of truth - also read by
+# flake.nix) so .gitea/workflows/version-bump.yml only ever has to touch one
+# file, rather than a "VERSION X.Y" grep/sed that can't tell this project's
+# own version apart from cmake_minimum_required's version above.
+file(STRINGS "${CMAKE_CURRENT_SOURCE_DIR}/.env" HYPR_CHROME_ENV_LINES)
+foreach(HYPR_CHROME_ENV_LINE IN LISTS HYPR_CHROME_ENV_LINES)
+    if(HYPR_CHROME_ENV_LINE MATCHES "^PLUGIN_VERSION=(.*)$")
+        set(HYPR_CHROME_PLUGIN_VERSION "${CMAKE_MATCH_1}")
+    endif()
+endforeach()
+
 project(hypr-chrome
     DESCRIPTION "Draws a chamfered HUD-style border frame with a title bar around each window."
-    VERSION 0.1
+    VERSION ${HYPR_CHROME_PLUGIN_VERSION}
 )
 
 # Match the Makefile's prior default (CXXFLAGS ?= -O2) so a bare `cmake -B