Stop runtime defaults from clobbering OMARCHY_PATH

The /etc/profile.d/omarchy.sh default for OMARCHY_PATH was being clobbered
by shipped config files that re-export the variable to the old script-mode
path. In package mode, every login shell, UWSM session, and Hyprland
autostart was effectively reversing the migration. Fix:

- default/bashrc: source from $OMARCHY_PATH/default/bash/rc; default
  OMARCHY_PATH to /usr/share/omarchy if not set (so the source line
  works in any boot order, including non-login interactive shells).
- default/bash/envs: change 'export OMARCHY_PATH=$HOME/...' to
  'export OMARCHY_PATH=${OMARCHY_PATH:-/usr/share/omarchy}' so the
  profile.d default and install.sh script-mode override both win.
- config/uwsm/env: same defaulting pattern. UWSM may not source
  profile.d, so the inline default still matters.
- default/hypr/autostart.lua: quickshell launched from $OMARCHY_PATH/shell
  (not literal $HOME path). The bash that runs hl.exec_cmd sees
  OMARCHY_PATH propagated from UWSM env.
- config/chromium-flags.conf: --load-extension hard-codes
  /usr/share/omarchy/... because chromium doesn't expand env vars or
  '~'. The omarchy-dev-link workflow symlinks /usr/share/omarchy to a
  checkout.
- bin/omarchy-install-browser: same chromium-flag path correction for
  brave-origin-beta-flags.conf.

default/bash/envs

@@ -6,6 +6,8 @@ export BAT_THEME=ansi
 export MANROFFOPT="-c"
 export MANPAGER="sh -c 'col -bx | bat -l man -p'"
 
-# Duplicated from .config/uwsm/env so SSH works too
-export OMARCHY_PATH=$HOME/.local/share/omarchy
-export PATH=$OMARCHY_PATH/bin:$PATH:$HOME/.local/bin
+# Duplicated from .config/uwsm/env so SSH works too.
+# /etc/profile.d/omarchy.sh defaults OMARCHY_PATH to /usr/share/omarchy.
+# Honor whatever's already set (so install.sh's script-mode override wins).
+export OMARCHY_PATH="${OMARCHY_PATH:-/usr/share/omarchy}"
+export PATH="$OMARCHY_PATH/bin:$PATH:$HOME/.local/bin"