Settings: Save and Cancel buttons instead of saving each change

Changes made in the Settings window show at once but are only kept with
the Save button, placed under the tab labels; it takes the accent color
while changes are waiting. Cancel goes back to the saved settings, and
closing the window (or the page) with unsaved changes asks first.

Tags are now sent as a whole list (PUT /api/tags, nothing saved when one
is invalid, the error gives its position) and "Restore default tags"
shows GET /api/tags/defaults until Save.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

README.md

@@ -351,8 +351,11 @@ From the command line: `curl -o logs.csv 'localhost:8080/api/export.csv?q=error&
 
 ## Settings
 
-The gear icon opens the settings, organized in tabs. Everything except color tags is
-remembered per browser.
+The gear icon opens the settings, organized in tabs. Changes show at once but are only kept
+with the **Save** button, under the tab labels: it takes the accent color while changes are
+waiting. **Cancel** goes back to the saved settings, and closing the window with unsaved changes
+asks before dropping them. Localization and Appearance are remembered per browser; color tags
+and log sources are saved on the server. Purging and resetting column widths act at once.
 
 - **Localization**
   - *Language*: English or French.