feat(updater): gate auto-updater behind Cargo feature updater

Makes tauri-plugin-updater and tauri-plugin-process optional
dependencies enabled by a default-on `updater` Cargo feature.
Sandboxed packaging targets (Flatpak, Snap) will build with
`--no-default-features` to exclude the plugins entirely,
satisfying store policies that prohibit competing auto-updaters.

Changes:
- Cargo.toml: add `updater` feature; plugins are now optional deps
- lib.rs: gate plugin registration on the feature
- updates/mod.rs: is_auto_update_eligible() returns false when
  the feature is off (catch-all for unusual build configs)
- capabilities/default.json: remove updater/process permissions
- build.rs: emit capabilities/auto-updater.json only when the
  feature is enabled; remove it otherwise. The file is gitignored
  since it is fully build-driven.

cc

src-tauri/capabilities/default.json

@@ -36,8 +36,6 @@
     "os:default",
     "dialog:default",
     "clipboard-manager:default",
-    "clipboard-manager:allow-write-text",
-    "updater:default",
-    "process:allow-restart"
+    "clipboard-manager:allow-write-text"
   ]
 }