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/CHANGELOG.md

@@ -4,6 +4,16 @@ All notable GoSentry changes are recorded in this file.
 
 ## Unreleased
 
+**Settings:**
+
+- The **Logs directory** row gained an **Open** button that shows the folder in
+  the desktop file manager (Explorer on Windows, the XDG handler on Linux), so
+  reading a log file no longer means copying the path by hand. It opens the
+  path currently in the field — including an edit that has not been saved yet —
+  resolving a relative directory against the application folder exactly as the
+  store does. A folder that is missing (the logs directory is created on the
+  first run) or cannot be opened is reported in a dialog.
+
 **Job dialog:**
 
 - The **Arguments** placeholder now states the field's rule — one argument per