sway: extract start-swayidle.sh as single source of truth (fix caffeine)

caffeine.sh click was silently breaking idle lock. Two bugs that
compounded:

1. caffeine.sh inlined its own swayidle command (with swaylock -f -i
   , not lock-fancy.sh), drifted out of sync with sway/config
   which used lock-fancy.sh. Toggle off would restart swayidle with a
   different lock command than the rest of the session.

2. WAYLAND_DISPLAY isn't set in waybar's on-click context. When
   caffeine.sh called 'swaymsg output * power off' (part of the
   restart command) and later called swaylock / grim (via lock-fancy),
   those failed silently. swayidle itself can run without
   WAYLAND_DISPLAY but it can't actually monitor input/output, so it
   exits immediately. Result: user clicks caffeine off, flag clears,
   icon goes back to 'inactive', but auto-lock is silently dead.
   The user thinks they turned caffeine off. They didn't. They're
   just unprotected.

Fix:
- new start-swayidle.sh holds the canonical swayidle command
- sway/config: exec $HOME/.config/sway/start-swayidle.sh
- caffeine.sh: killall swayidle; start-swayidle.sh &
- start-swayidle.sh probes /proc/*/environ for a Wayland client's
  env (mako always has it) and exports WAYLAND_DISPLAY /
  DBUS_SESSION_BUS_ADDRESS / XDG_RUNTIME_DIR / DISPLAY if missing
  before exec'ing swayidle

Single source of truth + env restoration = click works on every box
that has the same Wayland-capable process tree (mako, swaybar, etc.),
no matter who/what is calling the script.

Verified on tadbit: pre-fix toggle off killed swayidle permanently.
Post-fix: toggle off restarts swayidle successfully, env inherited
from mako's /proc/PID/environ.

dot_config/sway/start-swayidle.sh

@@ -0,0 +1,57 @@
+#!/bin/bash
+# start-swayidle.sh — canonical swayidle startup for the Sway session.
+#
+# Single source of truth for the swayidle command. Invoked from:
+#   1. ~/.config/sway/config:         exec $HOME/.config/sway/start-swayidle.sh
+#   2. ~/.config/sway/caffeine.sh:    after killall swayidle, to re-arm
+#
+# Why a wrapper instead of inlining the command in sway/config? Because
+# caffeine.sh needs to RESTART swayidle after killing it, and the restart
+# must use the same timeouts and lock command as the original. Hardcoding
+# it twice drifts (see git log — swayidle line in caffeine.sh got out of
+# sync with the one in sway/config, so toggling caffeine off would
+# silently change your lock command and timeout values).
+#
+# Env: this script is run by:
+#   - sway at session start, with the full Wayland/Sway env (correct)
+#   - caffeine.sh from waybar's on-click, with NO Wayland env (broken — see below)
+# When run from caffeine.sh, swayidle needs WAYLAND_DISPLAY to talk to the
+# compositor. We probe /proc for the env of an already-running Wayland client
+# (mako, swaybar, foot, etc.) and inherit WAYLAND_DISPLAY / DISPLAY /
+# DBUS_SESSION_BUS_ADDRESS / XDG_RUNTIME_DIR from it. This is the same
+# trick `dbus-update-activation-environment` uses internally, just per-process.
+#
+# Returns: never. Runs as the foreground process; sway's exec replaces the
+# shell with this script, and swayidle takes over. When started from
+# caffeine.sh, the script is backgrounded (&) and this function returns.
+set -e
+
+# --- 1. Restore Wayland env if missing -----------------------------------
+# The bug we're fixing: clicking the caffeine button runs this script
+# from waybar's on-click context, which doesn't inherit sway's env.
+# swayidle and lock-fancy.sh both call swaymsg/grim which need
+# WAYLAND_DISPLAY — without it, swayidle silently exits and the user
+# has NO idle lock at all (the worst possible failure mode: looks normal
+# in the bar, but auto-lock is silently dead).
+if [[ -z "$WAYLAND_DISPLAY" && -z "$SWAYSOCK" ]]; then
+    # Find any running Wayland client and steal its env. mako (notification
+    # daemon) is always present and always has the right env.
+    for pid in /proc/[0-9]*; do
+        [[ -r "$pid/environ" ]] || continue
+        candidate=$(tr '\0' '\n' < "$pid/environ" 2>/dev/null | grep -E '^(WAYLAND_DISPLAY|DBUS_SESSION_BUS_ADDRESS|XDG_RUNTIME_DIR|DISPLAY)=' || true)
+        if [[ -n "$candidate" ]]; then
+            export $candidate
+            break
+        fi
+    done
+fi
+
+# --- 2. Start swayidle ----------------------------------------------------
+# If invoked from sway's `exec` line (config), swayidle replaces this shell
+# and runs forever. If invoked from caffeine.sh (backgrounded with &), it
+# runs as a child of the parent shell and caffeine.sh returns immediately.
+exec swayidle -w \
+    timeout 300 "$HOME/.config/sway/lock-fancy.sh" \
+    timeout 600 'swaymsg "output * power off"' \
+    resume 'swaymsg "output * power on"' \
+    before-sleep "$HOME/.config/sway/lock-fancy.sh"