Implement TUI

[thesouldev]

Summary

This PR adds an interactive Textual TUI for managing Google Tasks, launched by running gtask with no arguments:

• Added a two-pane gruvbox TUI (tui.py, tui_widgets.py, tui.tcss) with a Lists sidebar and a task pane, plus keyboard-first navigation, mouse support, search, cursor wrap, and a r refresh that refetches all lists and tasks
• Added a docked add/edit panel with inline fields, keyboard-driven due (today/tomorrow/custom) and list pickers, and the ability to move a task to another list on edit
• Added inline list management (new, rename, delete) and made a/e/x context-aware by focused pane, so they target tasks in the task pane and lists in the sidebar
• Added subtask support where Shift+A adds a subtask under the selected task, rendered nested with ├/└ connectors and a done/total badge
• Added an all-caught-up celebration when every task due today is done, and hid completed tasks in a list unless completed today
• Extracted shared presentation helpers (due labels, tree ordering, link and colour helpers) into view.py and reused them in the CLI
• Fixed gtask login to fall back to a fresh browser login when the cached refresh token is revoked