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/Cargo.toml

@@ -16,6 +16,13 @@ path = "src/main.rs"
 [build-dependencies]
 tauri-build = { version = "2", features = [] }
 
+[features]
+default = ["updater"]
+# Enables the in-app auto-updater (tauri-plugin-updater + tauri-plugin-process).
+# Disabled for sandboxed packaging targets (Flatpak, Snap) which have their own
+# update channels and whose store policies prohibit competing updaters.
+updater = ["dep:tauri-plugin-updater", "dep:tauri-plugin-process"]
+
 [dependencies]
 # QBZ Crates (extracted core functionality)
 qbz-models = { path = "../crates/qbz-models" }
@@ -39,8 +46,8 @@ tauri-plugin-opener = "2"
 tauri-plugin-os = "2"
 tauri-plugin-single-instance = { version = "2", features = ["deep-link"] }
 tauri-plugin-deep-link = "2"
-tauri-plugin-updater = "2"
-tauri-plugin-process = "2"
+tauri-plugin-updater = { version = "2", optional = true }
+tauri-plugin-process = { version = "2", optional = true }
 
 # Serialization
 serde = { version = "1", features = ["derive"] }