feat: add Mattermost notification support

Add mmost:// and mmosts:// (secure) Apprise URL support for Mattermost
incoming webhooks. Supports optional botname override and custom paths.

- Add sendMattermost() function following existing notification patterns
- Update NotificationModal with Mattermost in examples and description

src/routes/settings/notifications/NotificationModal.svelte

@@ -414,14 +414,15 @@
 							placeholder="gotify://hostname/app-token
 discord://webhook_id/webhook_token
 slack://token_a/token_b/token_c
+mmost://hostname/webhook-token
 tgram://bot_token/chat_id
 ntfy://my-topic
 pushover://user_key/api_token
 jsons://hostname/webhook/path"
-							class="flex min-h-[220px] w-full rounded-md border border-input bg-transparent px-3 py-2 text-sm shadow-sm placeholder:text-muted-foreground focus-visible:outline-none focus-visible:ring-1 focus-visible:ring-ring"
-						></textarea>
-						<p class="text-xs text-muted-foreground">
-							Supports Gotify (gotify:// or gotifys:// for HTTPS), Discord, Slack, Telegram, ntfy, Pushover, and generic JSON webhooks.
+						class="flex min-h-[220px] w-full rounded-md border border-input bg-transparent px-3 py-2 text-sm shadow-sm placeholder:text-muted-foreground focus-visible:outline-none focus-visible:ring-1 focus-visible:ring-ring"
+					></textarea>
+					<p class="text-xs text-muted-foreground">
+						Supports Gotify (gotify:// or gotifys:// for HTTPS), Discord, Slack, Mattermost (mmost:// or mmosts://), Telegram, ntfy, Pushover, and generic JSON webhooks.
 						</p>
 					</div>
 				</div>