fix: unified message input, dropdowns, comboboxes and more

[adamleithp]

NOTE

• I haven't tested onboarding as i can't get past the github authorization flow!

Questionable design decisions

• branch dropdown in header row... since i cleaned up the message input it needed to go somewhere and it makes sense to me it should live with diff and pr in title row BUT it takes a lot of space... I suppose it could be max-width+truncate+tooltip
• Please note i'm reducing the button roundedness of Quill, so it's going to soon visually fit.

Summary

• Unified prompt input: Replace MessageEditor and TaskInputEditor with a single PromptInput component using InputGroup from quill for a consistent, tighter input layout across new-task and in-session views
• Mode selector dropdown: Replace shift+tab mode cycling with an explicit ModeSelector dropdown in the prompt input toolbar, showing icons and labels for each mode (plan, code, review, auto-accept, etc.)
• Header row branch selector: Move branch selector into the header row for active tasks, alongside diff stats badge and git interaction controls
• Dropdown/combobox consistency: Migrate FolderPicker, GitHubRepoPicker, BranchSelector, EnvironmentSelector, WorkspaceModeSelect, ModelSelector, ReasoningLevelSelector, and DiffSettingsMenu to use quill DropdownMenu/Combobox components with consistent sizing and styling
• Button groups: Group related controls (repo + branch, workspace mode selectors) using quill ButtonGroup for visual cohesion
• Removed old components: Delete MessageEditor, TaskInputEditor, EditorToolbar, ModeIndicatorInput, and AttachmentMenu.css in favor of the new unified architecture

Test plan

• Create a new task — verify repo picker, branch selector, workspace mode, model selector, reasoning level, and mode dropdown all render and function
• Switch between plan/code/review/auto-accept modes via the mode dropdown
• Verify in-session prompt input shows model selector, mode dropdown, attachment menu, and send/cancel buttons

• Check header row shows branch and diff stats for active tasks
• Test cloud workspace flow — GitHub repo picker, cloud branch selector, environment selector

• Test bypass permissions warning text updated in settings

• Verify tutorial onboarding flow still works with the new PromptInput