Standardise across machines; add genuinely-customised app config

Two passes, both aimed at a consistent experience rather than a faithful
snapshot of each machine's accidents.

Standardisation. Audited every host-layer config present on more than one
machine and asked whether the difference was intent or drift:
  - mpv was byte-identical; swayimg differed only because system76 still
    had the shipped default and the work laptop had the real config; bob
    differed by one setting the work laptop was simply missing.
  - user-dirs.dirs disagreed with BOTH machines and with zshenv, which has
    said $HOME/downloads all along. Standardised on that.
  - mimeapps.list is now the union of both machines' associations.
  - swaync's config and style are shared; only wallust's generated
    colors.css stays per host, alongside hypr's and waybar's.
All of the above moved to common/.

Dropped as pure state rather than config: gtk-2.0 (file-chooser geometry),
pavucontrol.ini (window size), zoom.conf (a per-install random_key) and
zoomus.conf (panel widths). gtk-3.0 stays per host — its bookmarks encode
the two different home directories.

App config. GIMP, inkscape, FreeCAD, cura, OrcaSlicer, LightBurn and OBS
all hold real settings, so a curated subset of each is now tracked:
settings, keybindings and user content, but not session state, tool
options, device lists or window geometry. kicad is skipped — both library
tables hold a single entry, i.e. untouched defaults.

Content lives once in common/ even when only one machine has the app; the
host configs decide who gets the link. That is the distinction that lets
53% of link sources be shared while the NAS still does not get GIMP.

Two exclusions on credential grounds: obs-studio/plugin_config holds a
websocket server_password, and gh is dropped entirely because `gh auth
login` writes a token into hosts.yml — now also blocked by name.

common/OrcaSlicer/user/default/machine/Creality K2 Pro 0.4 nozzle - Copy.info

@@ -0,0 +1,5 @@
+sync_info = update
+user_id = 
+setting_id = 
+base_id = GM001
+updated_time = 1765390291

common/OrcaSlicer/user/default/machine/Creality K2 Pro 0.4 nozzle - Copy.json

@@ -0,0 +1,14 @@
+{
+    "from": "User",
+    "inherits": "Creality K2 Pro 0.4 nozzle",
+    "name": "Creality K2 Pro 0.4 nozzle - Copy",
+    "print_host": "10.0.0.34:4408",
+    "printer_extruder_id": [
+        "1"
+    ],
+    "printer_extruder_variant": [
+        "Direct Drive Standard"
+    ],
+    "printer_settings_id": "Creality K2 Pro 0.4 nozzle - Copy",
+    "version": "2.3.1.20"
+}