Better assistant interaction

[lemoony]

This PR introduces a significant UI overhaul for the SnipKit Assistant, replacing the previous wizard-based workflow with a unified chat-style interface.

Key Features:

• Unified chat interface with multiple modes (Input, Generating, ScriptReady, PostExecution)
• Real-time execution preview with spinner and captured output
• Modal overlays for parameter collection and save dialogs
• Chat history preservation across interaction cycles

[codecov]

Codecov Report

❌ Patch coverage is 72.95082% with 528 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.54%. Comparing base (511d98c) to head (b678825).
⚠️ Report is 46 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/ui/execution/viewer.go	0.00%	147 Missing ⚠️
internal/ui/assistant/chat/chat_unified.go	73.26%	123 Missing and 12 partials ⚠️
internal/ui/form/parameter_modal.go	67.61%	81 Missing and 10 partials ⚠️
internal/app/app_assistant.go	82.31%	19 Missing and 10 partials ⚠️
internal/ui/assistant/chat/chat.go	72.94%	19 Missing and 4 partials ⚠️
internal/ui/assistant/chat/save_modal.go	87.42%	17 Missing and 5 partials ⚠️
internal/ui/assistant/chat/modal_helpers.go	84.88%	9 Missing and 4 partials ⚠️
internal/ui/assistant/chat/api.go	0.00%	12 Missing ⚠️
internal/ui/assistant/chat/message.go	92.89%	7 Missing and 5 partials ⚠️
internal/ui/assistant/chat/overlay.go	90.40%	8 Missing and 4 partials ⚠️
... and 8 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #318      +/-   ##
==========================================
+ Coverage   60.12%   61.54%   +1.42%     
==========================================
  Files         124      136      +12     
  Lines        7330     9068    +1738     
==========================================
+ Hits         4407     5581    +1174     
- Misses       2766     3281     +515     
- Partials      157      206      +49     

Flag	Coverage Δ
Linux	61.40% <72.95%> (+1.45%)	⬆️
macOS	61.52% <72.95%> (+1.42%)	⬆️

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