perf: return raw bytes from GetItemContent instead of base64

The D-Bus interface previously returned content as a base64-encoded
string (type 's'), requiring the daemon to encode and GJS to decode
on every paste. D-Bus natively supports byte arrays (type 'ay'),
eliminating both encode and decode steps entirely.

Changes:
- dbus_service.rs: return (String, Vec<u8>); drop base64 calls
- dbus.js: content_b64 arg changed from type 's' to type 'ay'
- panel.js: remove GLib.base64_decode(); pass ay bytes directly
- Cargo.toml/Cargo.lock: remove base64 dependency (no longer used)

For a 1 MB text paste this avoids allocating a 1.33 MB intermediate
string and decoding it on the compositor main thread.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

strata@edu4rdshl.dev/dbus.js

@@ -30,9 +30,9 @@ const STRATA_IFACE_XML = `
     </method>
 
     <method name="GetItemContent">
-      <arg type="s" direction="in"  name="id"/>
-      <arg type="s" direction="out" name="mime_type"/>
-      <arg type="s" direction="out" name="content_b64"/>
+      <arg type="s"  direction="in"  name="id"/>
+      <arg type="s"  direction="out" name="mime_type"/>
+      <arg type="ay" direction="out" name="content"/>
     </method>
 
     <method name="SetClipboard">

strata@edu4rdshl.dev/ui/panel.js

@@ -471,8 +471,8 @@ export class StrataPanel {
 
     async _onItemActivated(id) {
         try {
-            const [mimeType, contentB64] = await this._proxy.GetItemContentAsync(id);
-            this._writeToClipboard(mimeType, GLib.base64_decode(contentB64));
+            const [mimeType, content] = await this._proxy.GetItemContentAsync(id);
+            this._writeToClipboard(mimeType, content);
         } catch (e) {
             console.error('[Strata] Paste error:', e);
         }