feat: open the logs folder from the Settings tab

Reading a log file meant copying the configured path out of Settings and
pasting it into a file manager. The Logs directory row now carries an Open
button beside Browse that reveals the folder directly.

The new src/platform/filemanager package holds the platform split — explorer
on Windows, xdg-open on Linux, an "unsupported" error elsewhere — and starts
the handler without waiting on it, since Explorer exits non-zero even after it
opens the window and blocking would stall the UI thread. A missing path, a
path that is a file, and a handler that will not start are all reported to the
user; the logs directory does not exist until the first run, so that case is
reachable.

The button opens whatever the field currently holds rather than the saved
config, so an edit can be checked before Save. Resolving a relative directory
against the application folder is the store's rule, so resolveConfiguredDir is
now exported as storage.ResolveConfiguredDir instead of being duplicated in
the UI.

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

docs/ARCHITECTURE.md

@@ -18,6 +18,7 @@ src/
   platform/
     autostart/          Manager interface + Windows (shortcut) and Linux (XDG) impls
     desktop/            display-scale helper (Linux only)
+    filemanager/        open a folder in the desktop file manager
     winproc/            hidden-window startup flags (Windows only)
   ui/                   Fyne windows, tabs, and dialogs; reads service via Events
 ```