refactor(owlry): wire UI to use IPC client instead of direct provider calls

The UI now uses a SearchBackend abstraction that wraps either:
- CoreClient (daemon mode): connects to owlry-core via IPC for search,
  frecency tracking, submenu queries, and plugin actions
- Local ProviderManager (dmenu mode): unchanged direct provider access

Key changes:
- New backend.rs with SearchBackend enum abstracting IPC vs local
- app.rs creates CoreClient in normal mode, falls back to local if
  daemon unavailable
- main_window.rs uses SearchBackend instead of ProviderManager+FrecencyStore
- Command execution stays in the UI (daemon only tracks frecency)
- dmenu mode path is completely unchanged (no daemon involvement)
- Added terminal field to IPC ResultItem for proper terminal launch
- Added PluginAction IPC request for plugin command execution

Cargo.lock

@@ -652,6 +652,17 @@ version = "0.8.21"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d0a5c400df2834b80a4c3327b3aad3a4c4cd4de0629063962b03235697506a28"
 
+[[package]]
+name = "ctrlc"
+version = "3.5.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e0b1fab2ae45819af2d0731d60f2afe17227ebb1a1538a236da84c93e9a60162"
+dependencies = [
+ "dispatch2",
+ "nix",
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "deranged"
 version = "0.5.8"
@@ -689,6 +700,8 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1e0e367e4e7da84520dedcac1901e4da967309406d1e51017ae1abfb97adbd38"
 dependencies = [
  "bitflags",
+ "block2",
+ "libc",
  "objc2",
 ]
 
@@ -2208,6 +2221,18 @@ dependencies = [
  "getrandom 0.2.17",
 ]
 
+[[package]]
+name = "nix"
+version = "0.31.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5d6d0705320c1e6ba1d912b5e37cf18071b6c2e9b7fa8215a1e8a7651966f5d3"
+dependencies = [
+ "bitflags",
+ "cfg-if",
+ "cfg_aliases",
+ "libc",
+]
+
 [[package]]
 name = "nom"
 version = "1.2.4"
@@ -2456,6 +2481,7 @@ name = "owlry-core"
 version = "0.5.0"
 dependencies = [
  "chrono",
+ "ctrlc",
  "dirs",
  "env_logger",
  "freedesktop-desktop-entry",