feat(terminal): add embedded PTY-backed terminal dock with per-tab sessions

Adds a built-in terminal dock that runs a real pseudo-terminal per repository tab.

- Backend: new src-tauri/src/terminal.rs implements TerminalState and Tauri commands
  terminal_open, terminal_write, terminal_resize and terminal_close using the
  portable-pty crate. Terminal output is emitted via "terminal:data" and exits
  via "terminal:exit". Includes unit tests for UTF-8 chunk handling. Cargo.toml
  updated to depend on portable-pty and main.rs registers the state/commands.
- Frontend: App.svelte/UI and app.css updated to show a resizable terminal dock,
  toggleable with Ctrl+` and persisted open/height state. TerminalPanel.svelte is
  lazy-loaded per repo tab (frontend provides session ids like repo:<path>).
  package.json adds @xterm/xterm and @xterm/addon-fit for the frontend terminal
  surface.
- Docs: CHANGELOG.md notes the new embedded terminal feature.

Behavior: each repository tab has its own PTY-backed shell started in the repo
cwd; the user's shell (SHELL/COMSPEC) is used so prompts, aliases and interactive
behavior work as in a normal terminal.

src-tauri/Cargo.lock

@@ -511,6 +511,12 @@ version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9330f8b2ff13f34540b44e946ef35111825727b38d33286ef986142615121801"
 
+[[package]]
+name = "cfg_aliases"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fd16c4719339c4530435d38e511904438d07cce7950afa3718a84ac36c10e89e"
+
 [[package]]
 name = "cfg_aliases"
 version = "0.2.1"
@@ -919,6 +925,12 @@ dependencies = [
  "tendril",
 ]
 
+[[package]]
+name = "downcast-rs"
+version = "1.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "75b325c5dbd37f80359721ad39aca5a29fb04c89279657cffdda8736d0c0b9d2"
+
 [[package]]
 name = "dpi"
 version = "0.1.2"
@@ -981,7 +993,7 @@ dependencies = [
  "rustc_version",
  "toml 1.1.2+spec-1.1.0",
  "vswhom",
- "winreg",
+ "winreg 0.55.0",
 ]
 
 [[package]]
@@ -1099,6 +1111,17 @@ dependencies = [
  "rustc_version",
 ]
 
+[[package]]
+name = "filedescriptor"
+version = "0.8.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e40758ed24c9b2eeb76c35fb0aebc66c626084edd827e07e1552279814c6682d"
+dependencies = [
+ "libc",
+ "thiserror 1.0.69",
+ "winapi",
+]
+
 [[package]]
 name = "filetime"
 version = "0.2.29"
@@ -1475,6 +1498,7 @@ dependencies = [
  "commit_ai",
  "keyring",
  "log",
+ "portable-pty",
  "reqwest 0.12.28",
  "serde",
  "serde_json",
@@ -2160,6 +2184,12 @@ dependencies = [
  "zeroize",
 ]
 
+[[package]]
+name = "lazy_static"
+version = "1.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bbd2bcb4c963f2ddae06a2efc7e9f3591312473c50c6685e1f298068316e66fe"
+
 [[package]]
 name = "libappindicator"
 version = "0.9.0"
@@ -2378,6 +2408,18 @@ version = "1.0.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "650eef8c711430f1a879fdd01d4745a7deea475becfb90269c06775983bbf086"
 
+[[package]]
+name = "nix"
+version = "0.28.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ab2156c4fce2f8df6c499cc1c763e4394b7482525bf2a9701c9d79d215f519e4"
+dependencies = [
+ "bitflags 2.13.0",
+ "cfg-if",
+ "cfg_aliases 0.1.1",
+ "libc",
+]
+
 [[package]]
 name = "nix"
 version = "0.29.0"
@@ -2386,7 +2428,7 @@ checksum = "71e2746dc3a24dd78b3cfcb7be93368c6de9963d30f43a6a73998a9cf4b17b46"
 dependencies = [
  "bitflags 2.13.0",
  "cfg-if",
- "cfg_aliases",
+ "cfg_aliases 0.2.1",
  "libc",
  "memoffset",
 ]
@@ -2399,7 +2441,7 @@ checksum = "cf20d2fde8ff38632c426f1165ed7436270b44f199fc55284c38276f9db47c3d"
 dependencies = [
  "bitflags 2.13.0",
  "cfg-if",
- "cfg_aliases",
+ "cfg_aliases 0.2.1",
  "libc",
 ]
 
@@ -3055,6 +3097,27 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "portable-pty"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b4a596a2b3d2752d94f51fac2d4a96737b8705dddd311a32b9af47211f08671e"
+dependencies = [
+ "anyhow",
+ "bitflags 1.3.2",
+ "downcast-rs",
+ "filedescriptor",
+ "lazy_static",
+ "libc",
+ "log",
+ "nix 0.28.0",
+ "serial2",
+ "shared_library",
+ "shell-words",
+ "winapi",
+ "winreg 0.10.1",
+]
+
 [[package]]
 name = "potential_utf"
 version = "0.1.5"
@@ -3163,7 +3226,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0c1a41e437b6bbd489372cd4971de128e85c855f56c57f283d20ff016cf7c0a8"
 dependencies = [
  "bytes",
- "cfg_aliases",
+ "cfg_aliases 0.2.1",
  "pin-project-lite",
  "quinn-proto",
  "quinn-udp",
@@ -3203,7 +3266,7 @@ version = "0.5.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "addec6a0dcad8a8d96a771f815f0eaf55f9d1805756410b39f5fa81332574cbd"
 dependencies = [
- "cfg_aliases",
+ "cfg_aliases 0.2.1",
  "libc",
  "once_cell",
  "socket2",
@@ -3901,6 +3964,17 @@ dependencies = [
  "syn 2.0.118",
 ]
 
+[[package]]
+name = "serial2"
+version = "0.2.38"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b16809bc35793b19ce4e0c53924bc0dce3937f15487997cfdaed936004180730"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "serialize-to-javascript"
 version = "0.1.2"
@@ -3954,6 +4028,22 @@ dependencies = [
  "digest",
 ]
 
+[[package]]
+name = "shared_library"
+version = "0.1.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5a9e7e0f2bfae24d8a5b5a66c5b257a83c7412304311512a0c054cd5e619da11"
+dependencies = [
+ "lazy_static",
+ "libc",
+]
+
+[[package]]
+name = "shell-words"
+version = "1.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dc6fe69c597f9c37bfeeeeeb33da3530379845f10be461a66d16d03eca2ded77"
+
 [[package]]
 name = "shlex"
 version = "2.0.1"
@@ -4027,7 +4117,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "18051cdd562e792cad055119e0cdb2cfc137e44e3987532e0f9659a77931bb08"
 dependencies = [
  "bytemuck",
- "cfg_aliases",
+ "cfg_aliases 0.2.1",
  "core-graphics 0.24.0",
  "foreign-types 0.5.0",
  "js-sys",
@@ -5954,6 +6044,15 @@ dependencies = [
  "memchr",
 ]
 
+[[package]]
+name = "winreg"
+version = "0.10.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "80d0f4e272c85def139476380b12f9ac60926689dd2e01d4923222f40580869d"
+dependencies = [
+ "winapi",
+]
+
 [[package]]
 name = "winreg"
 version = "0.55.0"