Extends the existing terminal pane tree system to editor tabs so users
can view different files side by side.

- Cmd/Ctrl+D / Cmd/Ctrl+Shift+D split editor pane right / down
- Cmd/Ctrl+[ / Cmd/Ctrl+] cycle focus between editor panes
- Cmd/Ctrl+W with multiple panes closes the active pane only
- Clicking a file in the explorer loads it into the focused split pane
  (falls back to new tab when the pane has unsaved changes)

No existing behavior changed. Single-pane editor tabs and all terminal
behavior are identical to before.

New unit tests cover split, close, and focus-after-close tree transforms
for editor pane trees (src/modules/tabs/lib/editorPanes.test.ts).

Register file associations for common text/source/config types so Terax
appears in the OS "Open With" menu. Opening a file launches (or focuses)
the app, opens the file in an editor tab, and sets its parent folder as
the workspace.

- tauri.conf.json: bundle.fileAssociations (role Editor)
- lib.rs: accept a file launch arg (workspace = its parent); add
LaunchFile state + get_launch_file command; handle macOS
RunEvent::Opened (authorize parent, seed launch state, emit
"terax:open-file") for warm start and the cold-start race
- launchDir.ts: consumeLaunchFile()
- App.tsx: drain get_launch_file + listen "terax:open-file" -> open file

Register OS file associations so Terax appears in "Open With" for common
text/source/config files. Opening a file opens it in an editor tab and
makes its parent folder the workspace.

Today Terax only accepts a **directory** launch argument — you can't
hand
it a file from the file manager. Closes crynta#979.

- `tauri.conf.json`: `bundle.fileAssociations` (role `Editor`) for ~55
exts.
- `src-tauri/src/lib.rs`: launch-arg parsing now accepts a file
(workspace
= parent dir); added `LaunchFile` state + `get_launch_file` command;
added
a macOS `RunEvent::Opened` handler — GUI launches deliver Open-With
files
via the Apple open-files event, not argv — which authorizes the parent,
  seeds the drain-once launch state, and emits `terax:open-file` (covers
  warm start and the cold-start race).
- `src/lib/launchDir.ts`: `consumeLaunchFile()`.
- `src/app/App.tsx`: mount effect drains `get_launch_file` and listens
for
  `terax:open-file`, authorizes the parent, then opens the file.
  `openFileTab` dedupes by path, so double-delivery can't double-open.

- `pnpm check-types` clean, `pnpm lint` clean, `pnpm test` clean.
- `cargo clippy --all-targets --locked -- -D warnings` clean, `cargo
nextest run` clean.
- `pnpm tauri build`, launched via macOS Finder "Open With" and `open -a
<file>`:
  app launches, opens the file, no crash.

- [x] `pnpm lint` clean
- [x] `pnpm check-types` clean
- [x] `pnpm test` clean
- [x] Manual smoke-test of the affected feature
- [x] `cargo clippy --all-targets --locked -- -D warnings` clean
- [x] `cargo nextest run --locked` clean
- [x] Platforms tested: macOS

- The macOS `Opened` handler also seeds `LaunchDir` so the workspace
root
follows the file's folder on cold start; residual race if `Opened` fires
  after the webview polls is mitigated by the emit + listener.
- Linux/Windows **warm**-start ("Open With" into an already-running
instance) needs `tauri-plugin-single-instance` — not added here.
Cold-start
  via argv is wired but only manually tested on macOS.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

* **New Features**
* Added support for opening the app with a specific file (not just a
folder).
* Enhanced “Open With” handling on macOS to open the launched file
directly.
* Updated file associations so more text-based extensions open in Terax.
* **Bug Fixes**
* Improved warm-start and cold-start launch behavior to keep the correct
file active.
* Normalized Windows path separators for more reliable file-opening
behavior.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->