chore: add root .gitignore and untrack auto-generated cache files

The repo had no root-level .gitignore, so .dart_tool/, ephemeral plugin
symlinks and build artifacts were being tracked across the workspace.
Every flutter pub get regenerated them and they polluted PRs as recurring
noise.

- Add root .gitignore covering .dart_tool/, ephemeral plugin symlinks,
  build/, coverage/, IDE files
- Untrack ~39 existing cache files across root, modules and packages

apps/mobile_app/linux/flutter/ephemeral/.plugin_symlinks/file_selector_linux

@@ -1 +0,0 @@
-/Users/juliandalcalaf/.pub-cache/hosted/pub.dev/file_selector_linux-0.9.4/

apps/mobile_app/linux/flutter/ephemeral/.plugin_symlinks/image_picker_linux

@@ -1 +0,0 @@
-/Users/juliandalcalaf/.pub-cache/hosted/pub.dev/image_picker_linux-0.2.2/

apps/mobile_app/linux/flutter/ephemeral/.plugin_symlinks/path_provider_linux

@@ -1 +0,0 @@
-/Users/juliandalcalaf/.pub-cache/hosted/pub.dev/path_provider_linux-2.2.1/

apps/mobile_app/linux/flutter/ephemeral/.plugin_symlinks/share_plus

@@ -1 +0,0 @@
-/Users/juliandalcalaf/.pub-cache/hosted/pub.dev/share_plus-10.1.4/

apps/mobile_app/linux/flutter/ephemeral/.plugin_symlinks/shared_preferences_linux

@@ -1 +0,0 @@
-/Users/juliandalcalaf/.pub-cache/hosted/pub.dev/shared_preferences_linux-2.4.1/

apps/mobile_app/linux/flutter/ephemeral/.plugin_symlinks/url_launcher_linux

@@ -1 +0,0 @@
-/Users/juliandalcalaf/.pub-cache/hosted/pub.dev/url_launcher_linux-3.2.2/