feat: server list sidebar + notifications + new icon

- Added server list sidebar (GtkPaned) showing saved servers with
  name, user@host:port detail. Double-click to connect.
- Added '+' button in sidebar to add new servers
- Added notification bar (animated GtkRevealer) for status messages:
  'Server saved', 'Server connected', 'Server disconnected',
  'Server removed', 'Key generated', 'Key imported', 'Key deleted'
  Auto-hides after 4 seconds.
- Changed icon to terminal-style PNG + SVG (scalable)
- Server list updates automatically when servers are added/removed

Layout: [Sidebar: Servers] | [Notebook: Terminal Tabs]

icons/x3-ssh.svg

@@ -0,0 +1,18 @@
+<?xml version="1.0" standalone="no"?>
+<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20010904//EN"
+ "http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd">
+<svg version="1.0" xmlns="http://www.w3.org/2000/svg"
+ width="256.000000pt" height="256.000000pt" viewBox="0 0 256.000000 256.000000"
+ preserveAspectRatio="xMidYMid meet">
+<metadata>
+Created by potrace 1.16, written by Peter Selinger 2001-2019
+</metadata>
+<g transform="translate(0.000000,256.000000) scale(0.100000,-0.100000)"
+fill="#000000" stroke="none">
+<path d="M0 1280 l0 -1280 1280 0 1280 0 0 1280 0 1280 -1280 0 -1280 0 0
+-1280z m705 725 l140 -62 3 -51 c2 -29 1 -52 -1 -52 -2 0 -74 -29 -160 -65
+l-157 -64 0 52 0 53 93 33 c50 19 96 37 100 41 4 4 -37 24 -92 45 l-101 38 0
+53 c0 47 2 53 18 47 9 -4 80 -34 157 -68z m455 -345 l0 -50 -160 0 -160 0 0
+50 0 50 160 0 160 0 0 -50z"/>
+</g>
+</svg>